More Important Topics of Cylinder

Taxonomy

The name nurse shark is thought to be a corruption of nusse, a name which once referred to the catsharks of the family Scyliorhinidae. The nurse shark family name, Ginglymostomatidae, derives from the Greek: from ??????µ?? meaning hinge and st?µa meaning mouth.

Distribution and habitat

The nurse shark is a common inshore bottom-dwelling shark, found in tropical and subtropical waters on the continental and insular shelves. It is frequently found at

depths of 1 metre or less but may occur down to 12 m. Its common habitats are reefs, channels between mangrove islands and sand flats. It occurs in the Western Atlantic from Rhode Island down to southern Brazil;in the Eastern Atlantic from Cameroon to Gabon (and possibly ranges further north and south); in the Eastern Pacific from the southern Baja California to Peru; and around the islands of the Caribbean.

Behaviour and diet

Nurse sharks are nocturnal animals, spending the day in large inactive groups of up to 40 individuals. Hidden under submerged ledges or in crevices within the reef, the nurse sharks seem to prefer specific resting sites and will return to them each day after the night's hunting. By night, the sharks are largely solitary; they spend most of their time rifling through the bottom sediments in search of food. Their diet consists primarily of crustaceans, molluscs, tunicates, and other fish, particularly stingrays.

Their diet consists of a large number of marine invertebrates - spiny lobsters, crabs, shrimps, sea urchins, octopuses, squid, and marine snails and bivalves.

They are thought to take advantage of dormant fish which would otherwise be too fast for the sharks to catch; although their small mouths limit the size of prey items, the sharks have large throat cavities which are used as a sort of bellows valve. In this way nurse sharks are able to suck in their prey. Nurse sharks are also known to graze algae and coral.

Nurse sharks have been observed resting on the bottom with their bodies supported on their fins, possibly providing a false shelter for crustaceans which they then ambush and eat.

Reproduction

The mating season runs from late June to the end of July. Nurse sharks are ovoviviparous, meaning the eggs develop and hatch within the body of the female where the hatchlings develop further until live birth occurs. The gestation period is six months, with a typical litter of 21 - 28 pups.

The mating cycle is biennial, as it takes 18 months for the female's ovaries to produce another batch of eggs. The young nurse sharks are born fully developed at about 30 centimetres long in Ginglymostoma cirratum. They possess a spotted coloration which fades with age.

Interaction with humans

The nurse shark is not widely commercially fished, but because of its sluggish behaviour it is an easy target for local fisheries. Its skin is exceptionally tough and is prized for leather; its flesh consumed fresh and salted and its liver utilised for oil. It is not taken as a game fish. It has been reported in some unprovoked attacks on humans but is not generally perceived as a threat.

Reefs can be created artificially either by special construction or through deliberately sinking ships, but one can argue that these "reefs" are not real ones, as it is seldom the case that an artificial obstruction would be

created that is a hazard to shipping. These structures are usually created to enhance physical complexity on generally featureless sand bottoms in order to attract a diverse assemblage of organisms, especially fish. Thus, "artificial reef" is a misnomer, though firmly established as the term used for man-made underwater habitat structures.

Biotic reef types

There are a number of biotic reef types, including oyster reefs, but the most massive and widely distributed are tropical coral reefs. Although corals are major contributors to the framework and bulk material comprising a coral reef, the organisms most responsible for reef growth against the constant assault from ocean waves are calcarous algae, especially, although not entirely, species of coralline algae.

Geologic reef definition

Geologists define reefs and related terms (for example, bioherm, biostrome, carbonate mound) using the factors of depositional relief, internal structure, and biotic composition. There is no consensus on one universally applicable definition. A useful definition distinguishes reefs from mounds as follows.

Both are considered to be varieties of organosedimentary buildups: sedimentary features, built by the interaction of organisms and their environment, that have synoptic relief and whose biotic composition differs from that found on and beneath the surrounding sea floor. Reefs are held up by a macroscopic skeletal framework.

Coral reefs are an excellent example of this kind. Corals and calcareous algae grow on top of one another and form a three-dimensional framework that is modified in various ways by other organisms and inorganic processes. By contrast, mounds lack a macroscopic skeletal framework. Mounds are built by microorganisms or by organisms that don't grow a skeletal framework. A microbial mound might be built exclusively or primarily by cyanobacteria.

Cyanobacteria do not have skeletons and individuals are microscopic. Cyanobacteria encourage the precipitation or accumulation of calcium carbonate and can produce compositionally distinct sediment bodies that have relief on the seafloor. Cyanobacterial mounds were most abundant before the evolution of shelly macroscopic organisms, but they still exist today (stromatolites are microbial mounds with a laminated internal structure).

Bryozoans and crinoids, common contributors to marine sediments during the Mississippian (for example), produced a very different kind of mound. Bryozoans are small and the skeletons of crinoids disintegrate. However, bryozoan and crinoid meadows can persist over time and produce compositionally distinct bodies of sediment with depositional relief.

Geologic reef structures

Ancient reefs buried within stratigraphic sections are of considerable interest to geologists because they provide paleo-environmental information about the location in Earth's history.

In addition, reef structures within a sequence of sedimentary rocks provide a discontinuity which may serve as a trap or conduit for fossil fuels or mineralizing fluids to form petroleum or ore deposits. Corals, including some major extinct groups Rugosa and Tabulata, have been important reef builders through much of the Phanerozoic since the Ordovician period. However, other organism groups, such as calcifying algae, especially members of the red algae Rhodophyta, mollusks (especially the rudist bivalves during the Cretaceous period) have created massive structures at various times.

During the Cambrian period, the conical or tubular skeletons of Archaeocyatha,an extinct group of uncertain affinities (possibly sponges), built reefs. Other groups, such as the Bryozoa have been important interstitial organisms, living between the framework builders. The corals which build reefs today, the Scleractinia, arose after the Permian-Triassic extinction that wiped out the earlier rugose corals (as well as many other groups), and became increasingly important reef builders throughout the Mesozoic Era.

They may have arisen from a rugose coral ancestor. Rugose corals built their skeletons of calcite and have a different symmetry from that of the scleractinian corals, whose skeletons are aragonite. However, there are some unusual examples of well preserved aragonitic rugose corals in the late Permian. In addition, calcite has been reported in the initial post-larval calcification in a few scleractinian corals. Nevertheless, scleractinian corals (which arose in the middle Triassic) may have arisen from a non-calcifying ancestor independent of the rugosan corals (which disappeared in the late Permian).

Coral reefs:

Are aragonite structures produced by living organisms, found in shallow, tropical marine waters with little to no nutrients in the water. High nutrient levels such as that found in runoff from agricultural areas can harm the reef by encouraging the growth of algae.[1] In most reefs, the predominant organisms are stony corals, colonial cnidarians that secrete an exoskeleton of calcium carbonate (limestone).

The accumulation of skeletal material, broken and piled up by wave action and bioeroders, produces a massive calcareous formation that supports the living corals and a great variety of other animal and plant life. Although corals are found both in temperate and tropical waters, reefs are formed only in a zone extending at most from 30°N to 30°S of the equator; the reef-forming corals do not grow at depths of over 30 m (100 ft) or where the water temperature falls below 22 °C (72 °F).

Coral reef biology

The building blocks of coral reefs are the generations of reef-building corals, and other organisms that are composed of calcium carbonate. For example, as a coral head grows, it lays down a skeletal structure encasing each new polyp. Waves, grazing fish (such as parrotfish), sea urchins, sponges, and other forces and organisms break down the coral skeletons into fragments that settle into spaces in the reef structure. Many other organisms living in the reef community contribute their skeletal calcium carbonate in the same manner.

Coralline algae [i.e zooxanthelate,filamentous algae] are important contributors to the structure of the reef in those parts of the reef subjected to the greatest forces by waves (such as the reef front facing the open ocean). These algae contribute to reef-building by depositing limestone in sheets over the surface of the reef and thereby contributing to the structural integrity of the reef.

Reef-building or hermatypic corals are only found in the photic zone (above 50 m depth), the depth to which sufficient sunlight penetrates the water for photosynthesis to occur. The coral polyps do not photosynthesize, but have a symbiotic relationship with single-celled algae called zooxanthellae; these algal cells within the tissues of the coral polyps carry out photosynthesis and produce excess organic nutrients that are then used by the coral polyps.

Because of this relationship, coral reefs grow much faster in clear water, which admits more sunlight. Indeed, the relationship is responsible for coral reefs in the sense that without their symbionts, coral growth would be too slow for the corals to form impressive reef structures. Corals can get up to 90% of their nutrients from their zooxanthellae symbionts.

Although corals are found growing in most areas of a healthy coral reef, the elevation of the reef flat relative to sea level (and considering tidal range) imposes significant constraints on coral growth. In general, only a small number of hardy coral species can thrive on the reef flat, and these cannot grow above a certain height because the polyps can withstand only limited exposure to the air at low tide.

Of course some reef flats carry a meter or so of water over the surface, and then coral growth can be prolific. It is the upward growth of coralline algae on the outer part of the reef flat that ultimately results in an overall rise in the surface elevation of a reef, which typically slopes gently downward in towards the shore or lagoon and very steeply downward in the seaward direction.

Prolific growth of these algae is a response to water motion bringing in inorganic nutrients and removing waste products. The damaging effects of exposure at low tide on the algae is ameliorated somewhat by constantly breaking waves on the reef edge. Nonetheless, it is the case that mature reefs are in equilibrium with both sea level and wave regime with respect to their elevation, and excess production of limestone moves away from the margin to expand the reef laterally and fill in low areas.

The more prolific growths of corals are to be found in water deeper than where the bottom is exposed at low tides: on the frontal reef slope (forereef), in lagoons, and along reef channels that bisect the flat. Under conditions of clear, moving seawater, corals provide the bulk of the skeletal material comprising the reef and the structural complexity that results in a high diversity of reef associated fishes and invertebrates.

Coral reef formations

Coral reefs can take a variety of forms, defined in following;

Apron reef — short reef resembling a fringing reef, but more sloped; extending out and downward from a point or peninsular shore.

Fringing reef — reef that is directly attached to a shore or borders it with an intervening shallow channel or lagoon.

Barrier reef — reef separated from a mainland or island shore by a deep lagoon; see Great Barrier Reef.

Patch reef — an isolated, often circular reef, usually within a lagoon or embayment.

Ribbon reef — long, narrow, somewhat winding reef, usually associated with an atoll lagoon.

Table reef — isolated reef, approaching an atoll type, but without a lagoon.

Atoll reef — a more or less circular or continuous barrier reef extending all the way around a lagoon without a central island; see atoll.

Bank Reef — Bank reefs are larger than patch reefs and are linear or semi-circular in outline.

World-wide distribution of reefs

Coral reefs are estimated to cover 284,300 square kilometres, with the Indo-Pacific region (including the Red Sea, Indian Ocean, Southeast Asia and the Pacific) accounting for 91.9% of the total. Southeast Asia accounts for 32.3% of that figure, while the Pacific including Australia accounts for 40.8%. Atlantic and Caribbean coral reefs only account for 7.6% of the world total (Spalding et al., 2001).

Coral reefs are either restricted or absent from along the west coast of the Americas, as well as the west coast of Africa. This is due primarily to upwelling and strong cold coastal currents that reduce water temperatures in these areas (Nybakken, 1997). Corals are also restricted from off the coastline of South Asia from Pakistan to Bangladesh (Spalding et al., 2001).

They are also restricted along the coast around north-eastern South America and Bangladesh due to the release of vast quantities of freshwater from the Amazon and Ganges Rivers respectively.

Famous coral reefs and reef areas of the world include:

The Great Barrier Reef - largest coral reef system in the world, Queensland, Australia;

The Belize Barrier Reef - second largest in the world, Belize, Central America; and

The Red Sea Coral Reef - located off the coast of Egypt and Saudi Arabia.

Pulley Ridge - deepest photosynthetic coral reef, Florida

Many of the numerous reefs found scattered over the Maldives

Ecology and biodiversity

Coral reefs support an extraordinary biodiversity; although they are located in nutrient-poor tropical waters. The process of nutrient cycling between corals, zooxanthellae, and other reef organisms provides an explanation for why coral reefs flourish in these waters: recycling ensures that fewer nutrients are needed overall to support the community.

Cyanobacteria also provide soluble nitrates for the coral reef through the process of nitrogen fixation. Corals absorb nutrients, including inorganic nitrogen and phosphorus, directly from the water, and they feed upon zooplankton that are carried past the polyps by water motion (Castro and Huber, 2000).

Thus, primary productivity on a coral reef is very high. Producers in coral reef communities include the symbiotic zooxanthellae, coralline algae, and various seaweeds, especially small types called turf algae, although scientists disagree about the importance of these particular organisms (Castro and Huber, 2000).

Coral reefs are home to a variety of tropical or reef fishes, such as the colorful parrotfishes, angelfishes, damselfishes and butterflyfishes. Other fish groups found on coral reefs include groupers, snappers, grunts and wrasses. Over 4,000 species of fishes inhabit coral reefs (Spalding et al., 2001).

Reefs are also home to a large variety of other organisms, including sponges, Cnidarians (which includes some types of corals and jellyfish), worms, crustaceans (including shrimp, spiny lobsters and crabs), molluscs (including cephalopods), echinoderms (including starfish, sea urchins and sea cucumbers), sea squirts, sea turtles and sea snakes.

Aside from humans, mammals are rare on coral reefs, with visiting cetaceans such as dolphins being the main group. A few of these varied species feed directly on corals, while others graze on algae on the reef and participate in complex food webs (Castro and Huber, 2000; Spalding et al., 2001).

A number of invertebrates, collectively called cryptofauna, inhabit the coral rock substrate itself, either boring into the limestone surface or living in pre-existing voids and crevices. Those animals boring into the rock include sponges, bivalve molluscs, and Sipunculans. Those settling on the reef include many other species, particularly crustaceans and Polychaete worms (Nybakken, 1997).

Due to their vast biodiversity, many governments world-wide take measures to protect their coral reefs. In Australia, the Great Barrier Reef is protected by the Great Barrier Reef Marine Park Authority, and is the subject of many plans and pieces of legislation, including a Biodiversity Action Plan.

Threats to reefs

Human activity continues to represent the single greatest threat to coral reefs living in Earth's oceans. In particular, pollution and over-fishing are the most serious threats to these ecosystems. Physical destruction of reefs due to boat and shipping traffic is also a problem.

The live food fish trade has been implicated as a driver of decline due to the use of cyanide and other chemicals in the capture of small fishes. Finally, above normal water temperatures, due to climate phenomena such as El Niño and global warming, can cause coral bleaching.

According to The Nature Conservancy, if destruction increases at the current rate, 70% of the world’s coral reefs will have disappeared within 50 years. This loss would be an economic disaster for peoples living in the tropics. Hughes, et al, (2003), writes that "with increased human population and improved storage and transport systems, the scale of human impacts on reefs has grown exponentially.

For example, markets for fishes and other natural resources have become global, supplying demand for reef resources far removed from their tropical sources".

Currently researchers are working to determine the degree various factors impact the reef systems. The list of factors is long but includes the oceans acting as a carbon dioxide sink, changes in Earth's atmosphere, ultraviolet light, ocean acidification, biological virus, impacts of dust storms carrying agents to far flung reef systems, various pollutants, impacts of algal blooms and others... Reefs are threatened well beyond coastal areas and so the problem is broader than factors from land development and pollution though those are too causing considerable damage.

Land development and pollution

Extensive and poorly managed land development can threaten the survival of coral reefs. Within the last 20 years, once prolific mangrove forests, which absorb massive amounts of nutrients and sediment from runoff caused by farming and construction of roads, buildings, ports, channels, and harbors, are being destroyed. Nutrient-rich water causes fleshy algae and phytoplankton to thrive in coastal areas in suffocating amounts known as algal blooms.

Coral reefs are biological assemblages adapted to waters with low nutrient content, and the addition of nutrients favors species that disrupt the balance of the reef communities. Both the loss of wetlands and mangrove habitats are considered to be significant factors affecting water quality on inshore reefs.

Poor water quality has also been shown to encourage the spread of infectious diseases among corals.

Copper, a common industrial pollutant, has been shown to interfere with the life history and development of coral polyps.

Live reef fish trade

The hobby of keeping saltwater aquaria has experienced an increase in world popularity since the 1990s. Beyond sales of aquaria, air pumps, food, medications and other supplies, the primary product of the aquarium industry is fish. However, the world market is limited in the diversity of collected species.

For example, among 4000 coral reef fish species, only 200–300 are exploited. Selection of species results from a demand for fish being highly colorful and being able to be maintained and fed in aquaria. The last point is very important in the choice of imported species.

Although a few fish species (e.g. Pomacentridae) can be reproduced in aquaria, 95% of exploited fish are directly collected in the coral environment. Intense sampling of coral reef fish, especially in South-East Asia (including Indonesia and the Philippines), has caused great damage to the environment.

A major catalyst of cyanide fishing is poverty within fishing communities. In areas like the Philippines where cyanide is regularly used to catch live aquarium fish, the percentage of the population below the poverty line is 40%. In such developing countries, a fisherman might resort to such unethical practices in order to prevent his or her family from starving.

Most, 80–90%, of aquarium fish exported from the Philippines are captured with sodium cyanide. This toxic chemical is dissolved in sea water and released into fish shelters. It has a rapid narcotic effect on fish, which are then easily captured. However, most fish collected with cyanide die a few months after capture from extensive liver damage. Moreover, other fish species that are not interesting for the aquarium market also die in the field.

Dynamite Fishing

Dynamite fishing is another extremely destructive method that fishermen use to harvest small fish. Sticks of dynamite, grenades, or home-made explosives are lit or activated and thrown in the water. Once the dynamite goes off the explosion brings about an underwater shockwave, causing the internal organs of fish to liquefy, killing them almost instantly.

A second blast is often set off after the first to kill any larger predators that are attracted to the initial kill of the smaller fish. This method of fishing not only kills the fish within the main blast area, but also claims the lives of many reef animals that are not edible or wanted. Also, many of the fish do not float to the surface to be collected, but sink to the bottom.

The blast also kills the corals in the area, eliminating the very structure of the reef, destroying the habitat for fish and other animals important for the maintenance of a healthy reef. Areas that used to be full of coral become deserts, full of coral rubble, dead fish and little else after dynamite fishing.

Coral bleaching

During the 1998 and 2004 El Niño weather phenomena, in which sea surface temperatures rose well above normal, many tropical coral reefs were bleached or killed. Some recovery has been noted in more remote locations, but global warming could negate some of this recovery in the future. Toxins in the tissue are produced when the water temperatures climb, causing coral bleaching.[citation needed]

However, Ben McNeil of the University of New South Wales hypothesises that reefs are not in decline, and may exceed pre-industrial levels by as much as 35% by 2100, especially because of the positive influence of global warming. However, growth in some reefs due to global warming is expected to be offset by declines in other reefs, due to the comfortable temperature range for a coral being close to the temperature at which they bleach.[citation needed]

Some suggest that while reefs may die in certain areas, other areas will become habitable for corals, and form coral reefs.[9] Others yet point to data that suggests that the global temperature has never changed by more than a degree for a very long time. (See Global warming controversy).

Ocean acidification

The decreasing ocean surface pH is of increasing long-term concern for coral reefs. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans. Carbon dioxide gas dissolved in the ocean reacts with water to form carbonic acid, resulting in ocean acidification.

Ocean surface pH is estimated to have decreased from approximately 8.25 to 8.14 since the beginning of the industrial era, and it is estimated that it will drop by a further 0.3 - 0.4 units by 2100 as the ocean absorbs more anthropogenic CO2. Under normal conditions, the conditions for calcium carbonate production are stable in surface waters since the carbonate ion is at supersaturating concentrations.

However, as ocean pH falls, so does the concentration of this ion, and when carbonate becomes under-saturated, structures made of calcium carbonate are vulnerable to dissolution. Research has already found that corals experience reduced calcification or enhanced dissolution when exposed to elevated CO2.

Destruction worldwide

Southeast Asian coral reefs are at risk from damaging fishing practices (such as cyanide and blast fishing), overfishing, sedimentation, pollution and bleaching. A variety of activities, including education, regulation, and the establishment of marine protected areas are under way to protect these reefs. Indonesia, for example has nearly 33,000 square miles of coral reefs. Its waters are home to a third of the world’s total corals and a quarter of its fish species.

Indonesia's coral reefs are located in the heart of the Coral Triangle and have been victim to destructive fishing, unregulated tourism, and bleaching due to climatic changes. Data from 414 reef monitoring stations throughout Indonesia in 2000 found that only 6% of Indonesia’s coral reefs are in excellent condition, while 24% are in good condition, and approximately 70% are in poor to fair condition (2003 The Johns Hopkins University).

General estimates show approximately 10% of the coral reefs around the world are already dead. Problems range from environmental effects of fishing techniques, described above, to ocean acidification. Coral bleaching is another manifestation of the problem and is showing up in reefs across the planet.

Protection and restoration of reefs

Inhabitants of Ahus Island, Manus Province, Papua New Guinea, have followed a generations-old practice of restricting fishing in six areas of their reef lagoon. While line fishing is permitted, net and spear fishing are restricted based on cultural traditions. The result is that both the biomass and individual fish sizes are significantly larger in these areas than in places where fishing is completely unrestricted (Cinner et al. 2005).[19]

It is estimated that about 60% of the world’s reefs are at risk due to destructive, human-related activities. The threat to the health of reefs is particularly strong in Southeast Asia, where an enormous 80% of reefs are considered endangered.

Marine Protected Areas

One method of coastal reef management that has become increasingly prominent is the implementation of Marine Protected Areas (MPAs). MPAs have been introduced in Southeast Asia and elsewhere around the world to attempt to promote responsible fishery management and habitat protection. Much like the designation of national parks and wild life refuges, potentially damaging extraction activities are prohibited.

The objectives of MPAs are both social and biological, including restoration of coral reefs, aesthetic maintenance, increased and protected biodiversity, and economic benefits. Conflicts surrounding MPAs involve lack of participation, clashing views and perceptions of effectiveness, and funding.

Indonesia currently has nine MPAs, claiming a total 41,129 square kilometres of coastal waters are to be under protection. A study done on one of the more recently established MPAs in Indonesia showed the need for co-management when it comes to the success of managing MPAs. This collaborative approach emphasizes the cooperation and partnership between parties at the national, provincial, and local community level.

The coral reefs in the Philippines and Indonesia are disappearing rapidly due to dynamite and cyanide fishing. Between 1966 and 1986 the productivity of coral reefs in the Philippines dropped by one-third as the national population doubled (State of the Reefs).[citation needed] In Indonesia as well, over eighty percent of the coral reefs are under threat (The Jakarta Post).

These two locations are home to the world's most diverse range of corals. If the rate of destruction does not diminish, seventy percent of all the world's coral reefs will be gone in the next twenty-five to forty years (the Philippines).

Corals are marine animals from the class Anthozoa and exist as small sea anemone-like polyps, typically in colonies of many identical individuals.

The group includes the important reef builders that are found in tropical oceans, which secrete calcium carbonate to form a hard skeleton.

A coral "head", commonly perceived to be a single organism, is actually formed of thousands of individual but genetically identical polyps, each polyp only a few millimeters in diameter. Over thousands of generations, the polyps lay down a skeleton that is characteristic of their species.

A head of coral grows by asexual reproduction of the individual polyps. Corals also breed sexually by spawning, with corals of the same species releasing gametes simultaneously over a period of one to several nights around a full moon.

Although corals can catch plankton using stinging cells on their tentacles, these animals obtain most of their nutrients from symbiotic unicellular algae called zooxanthellae. Consequently, most corals depend on sunlight and grow in clear and shallow water, typically at depths shallower than 60 m (200 ft).

These corals can be major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland, Australia.

Other corals do not have associated algae and can live in much deeper water, such as in the Atlantic, with the cold-water genus Lophelia surviving as deep as 3000 m.[3] Corals have also been found off the coast of Washington State and the Aleutian Islands in Alaska.

Phylogeny

Corals belong to the class Anthozoa and are divided into two subclasses, depending on the number of tentacles or lines of symmetry, and a series of orders corresponding to their exoskeleton, nematocyst type, and mitochondrial genetic analysis.[1][2][4] Those with eight tentacles are called octocorallia or Alcyonaria and comprise soft corals, sea fans and sea pens.

Those with more than eight in a multiple of six are called hexacorallia or Zoantharia. This group includes reef-building corals (Scleractinians), sea anemones and zoanthids.

Anatomy

While a coral head appears to be a single organism, it is actually a head of many individual, yet genetically identical, polyps. The polyps are multicellular organisms that feed on a variety of small organisms, from microscopic plankton to small fish.

Polyps are usually a few millimeters in diameter, and are formed by a layer of outer epithelium and inner jellylike tissue known as the mesoglea. They are radially symmetrical with tentacles surrounding a central mouth, the only opening to the stomach or coelenteron, through which both food is ingested and waste expelled.

The stomach closes at the base of the polyp, where the epithelium produces an exoskeleton called the basal plate or calicle (L. small cup). This is formed by a thickened calciferous ring (annular thickening) with six supporting radial ridges (as shown below). These structures grow vertically and project into the base of the polyp allowing it to retreat into the exoskeleton for protection.

The polyp grows by extension of vertical calices which are occasionally septated to form a new, higher, basal plate. Over many generations this extension forms the large calciferous (Calcium containing) structures of corals and ultimately coral reefs.

Formation of the calciferous exoskeleton involves deposition of the mineral aragonite by the polyps from calcium ions they acquire from seawater. The rate of deposition, while varying greatly between species and environmental conditions, can be as much as 10 g / m² of polyp / day (0.3 ounce / sq yd / day). This is light dependent, with night-time production 90% lower than that during the middle of the day.

The polyp's tentacles trap prey using stinging cells called nematocysts. These are cells modified to capture and immobilize prey, such as plankton, by injecting poisons, firing very rapidly in response to contact.

These poisons are usually weak but in fire corals they are potent enough to harm humans. Nematocysts can also be found in jellyfish and sea anemones. The toxins injected by nematocysts immobilize or kill prey, which can then be drawn into the polyp's stomach by the tentacles through a contractile band of epithelium called the pharynx.

The polyps are interconnected by a complex and well developed system of gastrovascular canals allowing significant sharing of nutrients and symbiotes. In soft corals these range in size from 50-500 µm in diameter and to allow transport of both metabolites and cellular components.

Aside from feeding on plankton, many corals form a symbiotic relationship with a class of algae, zooxanthellae, of the genus Symbiodinium. Typically a polyp will harbour one particular species of algae. Via photosynthesis, these provide energy for the coral, and aid in calcification. The algae benefit from a safe environment, and use the carbon dioxide and nitrogenous waste produced by the polyp.

Due to the strain the algae can put on the polyp, stress on the coral often triggers ejection of the algae, known on a large scale as coral bleaching as it is the algae that give coral colour. This increases the polyps' chances of surviving stressful periods - they can regain the algae at a later time. If the conditions persist the polyps, and corals, will eventually die.

Reproduction

Sexual

Corals predominantly reproduce sexually, with 25% of hermatypic corals (stony corals) forming single sex (gonochoristic) colonies, whilst the rest are hermaphroditic. About 75% of all hermatypic corals "broadcast spawn" by releasing gametes - eggs and sperm - into the water to spread colonies over large distances.

The gametes fuse during fertilisation to form a microscopic larvum called a planula, typically pink and elliptical in shape; a moderately sized coral colony can form several thousands of these larva per year to overcome the huge odds against formation of a new colony.

The planula swims towards light, exhibiting positive phototaxis, to surface waters where they drift and grow for a time before swimming back down to locate a surface on which it can attach and establish a new colony. At many stages of this process there are high failure rates, and even though millions of gametes are released by each colony very few new colonies are formed.

The time from spawning to settling is often 2-3 days, but can be up to 2 months. The larva grows into a coral polyp and eventually becomes a coral head by asexual budding and growth, creating new polyps.

Corals that do not broadcast spawn are called brooders, with most non-stony corals displaying this characteristic. These corals release sperm but harbour the eggs, allowing larger, negatively buoyant, planulae to form which are later released ready to settle. The larva grows into a coral polyp and eventually becomes a coral head by asexual budding and growth to create new polyps.

Synchronous spawning is very typical on a coral reef and often, even when there are multiple species present, all the corals on the reef release gametes during the same night. This synchrony is essential so that male and female gametes can meet and form planula.

The cues that guide the release are complex, but over the short term involve lunar changes, sunset time, and possibly chemical signalling.[9] Synchronous spawning may have the result of forming coral hybrids, perhaps involved in coral speciation. In some places the coral spawn can be dramatic, usually occurring at night, where the usually clear water becomes cloudy with gametes.

Asexual

Within a head of coral the genetically identical polyps reproduce asexually to allow growth of the colony. This is achieved either through gemmation or budding or through division, both shown in the diagrams of Orbicella annularis. Budding involves a new polyp growing from an adult, whereas division forms two polyps each as large as the original.

Whole colonies can reproduce asexually through fragmentation, where a piece broken off a coral head and moved by wave action can continue to grow in a new location.

Coral reefs

The hermatypic, stony corals are often found in coral reefs, large calcium carbonate structures generally found in shallow, tropical water. Reefs are built up from coral skeletons and held together by layers of calcium carbonate produced by coralline algae. Reefs are extremely diverse marine ecosystems being host to over 4,000 species of fish, massive numbers of cnidarians, molluscs, crustaceans, and many other animals.

Environmental effects on coral

Corals are highly sensitive to environmental changes. Scientists have predicted that over 50% of the coral reefs in the world may be destroyed by the year 2030; as a result they are generally protected through environmental laws. A coral reef can easily be swamped in algae if there are too many nutrients in the water.

Coral will also die if the water temperature changes by more than a degree or two beyond its normal range or if the salinity of the water drops.

In an early symptom of environmental stress, corals expel their zooxanthellae; without their symbiotic unicellular algae, coral tissues become colorless as they reveal the white of their calcium carbonate skeletons, an event known as coral bleaching.

Many governments now prohibit removal of coral from reefs to reduce damage by divers. However, damage is still caused by anchors dropped by dive boats or fishermen. In places where local fishing causes reef damage, education schemes have been run to inform the population about reef protection and ecology.

The narrow niche that coral occupies, and the stony corals' reliance on calcium carbonate deposition, means they are very susceptible to changes in water pH. Ocean acidification, caused by dissolution of carbon dioxide in the water, is currently occurring due to an increase in atmospheric carbon dioxide.

This changes the balance of production and dissolution of calcium carbonate, leading to destruction of corals.

A combination of temperature changes, pollution, and overuse by divers and jewelry producers has led to the destruction of many coral reefs around the world. This has increased the importance of coral biology as a discipline.

Climatic variations can cause temperature changes that destroy corals. For example, during the 1997-98 warming event all the hydrozoan Millepora boschmai colonies near Panamá were bleached and died within six years - this species is now thought to be extinct.

Uses

Live corals

Local economies near major coral reefs benefit from an abundance of fish and octopus as a food source. Reefs also provide recreational scuba diving and snorkeling tourism. Unfortunately all these activities can also have deleterious effects, such as removal or accidental destruction of coral.

Red shades of coral are sometimes used as a gemstone, especially in Tibet. In vedic astrology, red coral represents Mars. Pure red coral is known as 'fire coral' and is very rare because of the demand for perfect fire coral in jewellery-making.

Ancient corals

Ancient coral reefs on land are often mined for lime or use as building blocks ("coral rag"), for example the Portland limestone of the Isle of Portland. Coral rag is an important local building material in places such as the east African coast.

Some coral species exhibit banding in their skeletons resulting from annual variations in their growth rate. In fossil and modern corals these bands allow geologists to construct year-by-year chronologies, a form of incremental dating, which can provide high-resolution records of past climatic and environmental changes when combined with geochemical analysis of each band.

Certain species of corals form communities called microatolls. The vertical growth of microatolls is limited by average tidal height. By analyzing the various growth morphologies, microatolls can be used as a low resolution record of patterns of sea level change.

Fossilized microatolls can also be dated using radioactive carbon dating to obtain a chronology of patterns of sea level change. Such methods have been used to used to reconstruct Holocene sea levels.

They are sometimes confused with pufferfish. Porcupinefish are closely related to pufferfishes but porcupinefish have spines on their body.

Porcupinefish have the ability to inflate their body by

swallowing water (or air) and become round like a ball. This increase in size (almost double vertically) reduces the range of potential predators to those with much bigger mouths.

A second defense mechanism is provided by the sharp spines, which radiate outwards when the fish is inflated. Some species are poisonous, having a tetrodotoxin in their skin and/or intestines. As a result, porcupinefish have few predators: adults are rarely eaten except by sharks and orcas, though juveniles are also preyed on by tuna and dolphins.

The average size of such damselfish is around 3 inches (8 centimetres). They are all marine, however, a couple of species are regularly found in the lower stretches of rivers in pure freshwater, and usually have bright colours. Some

species of damselfish are able to adapt well in an average aquarium, but others such as the white-spotted damselfish cannot. The diet of a damselfish can include small crustaceans, plankton, and algae.

Many species of damselfish are kept as aquaria, and live in tropical coral reefs; however, many also live in temperate climates. One example would be damsels inhabiting the coast of southern California and northern pacific Mexican coast.

A common function for Damselfish is as a biological stabilizer in new aquariums. The fish would live in the aquarium during its initial existence, and be used to allow the aquarium to biologically stabilize with beneficial bacteria.

This practice is viewed negatively by many aquarists because of the foul conditions the fish are subjected to and the fact that more humane methods to stabilize an aquarium exist.

Pterophyllum is a small genus of freshwater fish from the family Cichlidae known to most aquarists as angelfish. All Pterophyllum species originate from the Amazon River basin in tropical South America.

The three species of Pterophyllum are unusually shaped for cichlids being greatly laterally compressed, with round bodies and elongated triangular-shaped dorsal and anal fins. This body shape allows them to hide among roots and

plants, often on a vertical surface. Naturally occurring angelfish are frequently striped longitudinally, colouration which provides additional camouflage. Angelfish are ambush predators and prey on small fish and macroinvertebrates. All Pterophyllum species form monogamous pairs. Eggs are generally laid on a flattened leaf or submerged log. As is the case for other cichlids, brood care is highly developed.

vertebrate parasite to attack humans, as well as Mekong giant catfish, the largest reported freshwater fish. There are armour-plated types and also naked types, neither having scales.

Despite their common name, not all catfish have prominent barbels; what defines a fish as being in the order Siluriformes are in fact certain features of the skull and swimbladder.

Catfish are of considerable commercial importance; many of the larger species are farmed or fished for food, and some are exploited for sport fishing, including a kind known as noodling. Many of the smaller species, particularly the genus Corydoras, are important in the aquarium hobby.

Taxonomy

Catfish belong to a superorder called the Ostariophysi, which also includes the Cypriniformes, Characiformes, Gonorynchiformes and Gymnotiformes, a superorder characterized by the Weberian apparatus.

Some place Gymnotiformes as a sub-order of Siluriformes, however this is not as widely accepted. Currently, the Siluriformes are said to be the sister group to the Gymnotiformes, though this has been debated due to more recent molecular evidence.

As of 2006 there are 35 extant catfish families, and about 2,867 species have been described. This makes the catfish order the second or third most diverse vertebrate order; in fact, 1 out of every 20 vertebrate species is a catfish.

The taxonomy of catfishes is quickly changing. There is disagreement on the family status of certain groups; for example, Nelson (2006) lists Auchenoglanididae and Heteropneustidae as separate families, while the All Catfish Species Inventory (ACSI) includes them under other families.

Also, FishBase and the Integrated Taxonomic Information System lists Parakysidae as a separate family, while this group is included under Akysidae by both Nelson (2006) and ACSI. Many sources do not list the recently revised family Anchariidae. Thus, the actual number of families differs between authors.

The species count is in constant flux due to taxonomic work as well as description of new species. On the other hand, our understanding of catfishes should increase in the next few years due to work by the ACSI.

Except for Diplomystidae, the most primitive family of catfish, the relationship between the families is relatively uknown. Classifications of superfamilies varies. Many catfish families are classified into their own superfamilies. Only a few superfamilies contain more than one family.

In Nelson (2006) these are Loricarioidea (Amphillidae, Trichomycteridae, Nematogenyidae, Callichthyidae, Scoloplacidae, Astroblepidae, Loricariidae), Sisoroidea (Amblycipitidae, Akysidae, Sisoridae, Erethistidae, Aspredinidae), Doradoidea (Mochokidae, Doradidae, Auchenipteridae), Siluroidea (Siluridae, Malapteruridae, Auchenoglanididae, Chacidae, Plotosidae, Clariidae, Heteropneustidae), and Bagroidea (Austroglanididae, Claroteidae, Ariidae, Schilbeidae, Pangasiidae, Bagridae, and Pimelodidae).

In a recent phylogenetic analysis, however, alternative superfamilies with different constituent families were proposed. In this study, the superfamilies are Clarioidea (Clariidae, Heteropneustidae), Arioidea (Ariidae, Anchariidae), Pimelodoidea (Pimelodidae, Pseudopimelodidae, Heptapteridae, Conorhynchos), Ictaluroidea (Ictaluridae, Cranoglanididae), and Doradoidea (Doradidae, Auchenipteridae). Also, Sisoroidea, unlike that of Nelson, does not include Aspredinidae. The other remaining families are classified by themselves or grouped with other families or superfamilies in larger unranked clades.

The rate of description of new catfishes is at an all-time high. Between 2003 and 2005, over 100 species have been named, a rate three times faster than that of the past century.

In June, 2005, researchers named the newest family of catfish, Lacantuniidae, only the third new family of fish distinguished in the last 70 years (others being the coelacanth in 1938 and the megamouth shark in 1983). The new species in Lacantuniidae, Lacantunia enigmatica, was found in the Lacantun river in Chiapas, Mexico.

Evolution

A number of catfish fossils are known. The earliest known catfish are known from the late Campanian-early Maastrichtian of Argentina. Catfish fossils are known from every continent Australia. Fossils of the Eocene period have been found from Seymour Island in Antarctica.

The order dispersed early throughout the continents primarily through land bridges. Australian species of catfish are all species from families that can enter saltwater; these fish traveled to Australia this way, and then reverted to a freshwater lifestyle.

The catfish must have spread through Africa to Asia during the late Jurassic if they were to reach Asia. During the Cretaceous period, the rift between South America and Africa would be forming; this may explain the contrast in families between the two continents. Most of the freshwater catfish of the two continents appear to be completely unrelated.

Their relatively low diversity in Africa may explain why some primitive fish families coexist with them while they are absent in South America, where the more primitive fish may have been driven extinct. The earliest they could have spread into Central America was the late Miocene.

Distribution and habitat

Extant catfish species live in inland or coastal waters of every continent except Antarctica. Catfish have inhabitated all continents at one time or another. Catfish are most diverse in tropical South America, Africa, and Asia. More than half of all catfish species live in the Americas. They are the only ostariophysans that have entered freshwater habitats in Madagascar, Australia, and New Guinea.

They are found primarily in freshwater environments of all kinds, though most inhabit shallow, running water habitats. Some species even inhabit caves. ground in phreatic habitats. Numerous species from the families Ariidae and Plotosidae, and a few species from among the Aspredinidae and Bagridae, are also found in marine environments.

Ecology

Most catfish are benthic in nature, meaning they normally associate with the bottom of the water column.However, variety of other lifestyles are also represented among the catfishes. A few species are pelagic in nature. The candirú is a parasitic catfish that can attack humans. Panaque is a genus of catfishes that are the only fishes able to eat and digest wood.

Physical characteristics

Most catfish are adapted for a benthic lifestyle. In general, they are negatively buoyant, which means that they will usually sink rather than float due to a reduced gas bladder and a heavy, bony head. Catfish have a variety of body shapes, though most have a cylindrical body with a flattened ventrum to allow for benthic feeding.

A flattened head allows for digging through the substrate as well as perhaps serving as a hydrofoil. Most have a mouth that can expand to a large size and contains no incisiform teeth; catfish generally feed through suction or gulping rather than biting and cutting prey.

However, some families, notably Loricariidae and Astroblepidae, have a suckermouth that allows them to fasten themselves to objects in fast-moving water. Catfish also have a maxilla reduced to a support for barbels; this means that they are unable to protrude their mouths as other fish such as carp.

Catfish have no scales; their bodies are either naked or covered in bony plates called scutes. In some species, the mucus-covered skin is used in cutaneous respiration, where the fish breathes through its skin. They may have up to four pairs of barbels: nasal, maxillary (on each side of mouth), and two pairs of chin barbels, although pairs of barbels may be absent, depending on the species.

Because their barbels are more important in detecting food, the eyes on catfish are generally small. Like other ostariophysans, they are characterized by the presence of a Weberian apparatus. Their well-developed Weberian apparatus and reduced gas bladder allow for improved hearing as well as sound production.

All catfish, except members of Malapteruridae (electric catfish), possess a strong, hollow, bonified leading spine-like ray on their dorsal and pectoral fins. As a defense, these spines may be locked into place so that they stick outwards, which can inflict severe wounds.

In several species catfish can use these fin rays to deliver a stinging protein if the fish is irritated. This poison is produced by glandular cells in the epidermal tissue covering the spines. In members of the family Plotosidae, and of the genus Heteropneustes, this protein is so strong it may hospitalize humans unfortunate enough to receive a sting; in Plotosus lineatus, the stings may result in death.

Size

Catfish have one of the greatest range in size within a single order of bony fish. Catfish range in size and behavior from the heaviest, the Mekong giant catfish in Southeast Asia and the longest, the wels catfish of Eurasia, to detritivores (species that eat dead material on the bottom), and even to a tiny parasitic species commonly called the candiru, Vandellia cirrhosa.

Some of the smallest species of Aspredinidae and Trichomycteridae reach sexual maturity at only 10 millimetres (.4 in). Many catfish have a maximum length of under 12 cm.

The wels catfish, Silurus glanis, is the only native catfish species of Europe, besides the much smaller related Aristotle catfish found in Greece. Mythology and literature record wels catfish of astounding proportions, yet to be scientifically proved. The average size of the species is about 1.2 m to 1.6 m, and fish more than 2 m are very rare. The largest specimens on record measure more than 2.5 m in length and sometimes exceeded 100 kg.

The wels catfish was introduced to Britain, Italy, Spain, Greece and some other countries during the last century. The species has flourished in the warm lakes and rivers of Southern Europe. The River Danube, River Po in Italy and the River Ebro in Spain are famous for huge wels catfish, which grow up to 2 m. These habitats contain plenty of food and lack natural predators.

Ictalurus furcatus, in the Mississippi River on May 22, 2005 that weighed 56.25 kg (124 lb). The largest flathead catfish, Pylodictis olivaris, ever caught was in Independence, Kansas, weighing 56 kg (123 lb 9 oz).

However, these records pale in comparison to a giant Mekong catfish caught in northern Thailand in May 1, 2005 and reported to the press almost 2 months later, that weighed 293 kg (646 lb). This is the largest giant Mekong catfish caught, but only since Thai officials started keeping records in 1981. The giant Mekong catfish are not well studied since they live in developing countries and it is quite possible that they can grow even larger.

A Lionfish is any of several species of venomous marine fish in the genera Pterois, Parapterois, Brachypterois, Ebosia or Dendrochirus, of the family Scorpaenidae. The lionfish is also known as the Turkey Fish, Dragon Fish, and Scorpion Fish.

The lionfish are voracious predators. When they are hunting, they corner prey using their large fins and then

use their lightning quick reflexes to swallow the prey whole. They are notable for their extremely long and separated spines, and have a generally striped appearance, red, brown, orange, yellow, black, or white.

The group of fishes has been classified as a subfamily (Pteroinae) or as a tribe under Scorpaeninae (Pteroini).

While the hardiness and disease resistance of the lionfish make their care relatively simple, the venom of the spines is extremely painful, and lionfish are recommended for only the careful aquarist.

The lionfish is native to the tropical Indo-Pacific region of the world, but various species can be found worldwide. Due to a recent introduction, the lionfish has recently been spotted in the warmer coral regions of the Eastern Atlantic Ocean and Caribbean Sea. Successful breeding of the lionfish in captivity has not been reported.

Habitat

Lionfish are usually found in the Indo-Pacific, near and offshore coral and rocky reefs. There have also been sightings of lionfish in the Eastern Atlantic coast from Long Island to Florida. Lionfish can also be found in bays, estuaries, and even harbors. They show a clear preference for ledges, caves, and crevices, by day. Although they have been spotted a few times feeding during the day, it is believed that they are mostly nocturnal.

ancestors of echinoderms are believed to have had bilateral symmetry, and starfish do exhibit some superficial remnant of this body structure, which is evident in their larval pluteus forms. Starfish do not rely on a jointed, movable skeleton for support and locomotion (though they are protected by their skeleton), but instead possess a hydraulic water vascular system that aids in locomotion.

The water vascular system has many projections called tube feet, located on the ventral face of the starfish's arms, which function in locomotion and aid with feeding.

Distribution

There are about 1,800 living species of starfish, and they occur in all of the Earth's oceans. The greatest variety of starfish is found in the tropical Indo-Pacific. Areas known for their great diversity include the tropical-temperate regions around Australia, the tropical East Pacific, and the cold-temperate water of the North Pacific (California to Alaska).

Asterias is a common genus found in European waters and on the eastern coast of the United States; Pisaster, along with Dermasterias ("leather star"), are usually found on the western coast. Habitats could range from tropical coral reefs, kelp forests to deep-sea floor, although none of them live within the water column; all species of starfish found are living as benthos. Echinoderms need a delicate internal balance in their body; no starfish are found in freshwater environments.

External anatomy

Starfish are composed of a central disc from which arms sprout in pentaradial symmetry. Most starfish have five arms, however some have more or less; in fact some starfish can have different numbers of arms even within one species. The mouth is located underneath the starfish on the oral or ventral surface, while the anus is located on the top of the animal. The spiny upper surface covering the species is called the aboral or dorsal surface.

On the aboral surface there is a structure called the madreporite, a small white spot located slightly off-center on the central disc which acts as a water filter and supplies the starfish's water vascular system with water to move. Additional parts like cribriform organs present exclusively in Porcellanasteridae are used to generate current in the burrows made by these infaunal starfish.

Starfish, while having their own basic body plan, radiate diversely in shapes and colors and the morphology differs between each species; for example, a species of starfish may have rows of spines on their arms as means of protection. Ranging from nearly pentagonal (example: Indo-pacific cushion star, Culcita novaeguineae) to gracile stars like those on Zoroaster genus.

Starfish have a simple photoreceptor eyespot at the end of each arm. The eye is able to "see" only differences of light and dark, which is useful in detecting movement.

On the surface of the starfish, surrounding the spines, are small white objects known as pedicellariae. There are large numbers of these pedicellariae on the external body which serve to prevent encrusting organisms from colonizing the starfish. The radial canal which is across each arm of the starfish has tooth-like structures called ampullae, which surround the radial canal.

Patterns including mosaic-like tiles formed by ossicles, stripes, interconnecting net between spines, pustules with bright colors, mottles or spots. These mainly serve as camouflage or warning coloration displayed by many other marine animals as means of protection against predation. Several types of toxins and secondary metabolites have been extracted from several species of starfish and now being subjected into research worldwide for pharmacy or other uses such as pesticides.

Internal anatomy

The body cavity also contains the water vascular system that operates the tube feet, and the circulatory system, also called the hemal system. Hemal channels form rings around the mouth (the oral hemal ring), closer to the top of the starfish and around the digestive system (the gastric hemal ring). The axial sinus, a portion of the body cavity, connects the three rings. Each ray also has hemal channels running next to the gonads.

Digestion and excretion

Starfish digestion is carried out in two separate stomachs. They are called the cardiac stomach and the pyloric stomach. The cardiac stomach, which is a sack like stomach located at the center of the body may be everted—pushed out of the organism's body and used to engulf and digest food. Some species take advantage of the great endurance of their water vascular systems to force open the shells of bivalve mollusks such as clams and mussels, and inject their stomachs into the shells.

Once the stomach is inserted inside the shell it digests the mollusk in place. The cardiac stomach is then brought back inside the body, and the partially digested food is moved to the pyloric stomach. Further digestion occurs in the intestine and waste is excreted through the anus on the aboral side of the body.

Because of this ability to digest food outside of its body, the sea star is able to hunt prey that are much larger than its mouth would otherwise allow including arthropods, and even small fish in addition to mollusks.

Some echinoderms have been shown to live for several weeks without food under artificial conditions—it is believed that they may receive some nutrients from organic material dissolved in seawater.

Sea stars and other echinoderms have endoskeletons (internal skeletons), which is one of the reasons some scientists are led to beleive that echinoderms are very cosely related to chordates (animals with a hollow nerve chord that usually have vertebrae).

Nervous system

Echinoderms have rather complex nervous systems, but lack a true centralized brain. All echinoderms have a nerve plexus (a network of interlacing nerves), which lies within as well as below the skin. The esophagus is also surrounded by a number of nerve rings, which send radial nerves that are often parallel with the branches of the water vascular system.

The ring nerves and radial nerves coordinate the starfish's balance and directional systems. Although the echinoderms do not have many well-defined sensory inputs, they are sensitive to touch, light, temperature, orientation, and the status of water around them. The tube feet, spines, and pedicellariae found on starfish are sensitive to touch, while eyespots on the ends of the rays are light-sensitive.

Behavior

Diet

Most species are generalist predators, some eating bivalves like mussels, clams, and oysters; or any animal too slow to evade the attack (e.g. dying fish). Some species are detritivores, eating decomposed animal and plant material, or organic films attached to substrate. The others may consume coral polyps (the best-known example for this is the infamous Acanthaster planci), sponges or even suspended particles and planktons (starfish from the Order Brisingida).

The process of feeding or capture may or may not be aided by special parts; Pisaster brevispinus or Short-spined Pisaster from the west coast of America may use a set of specialized tube feet capable of extending itself deep into the soft substrata, hauling out the prey (usually clams) from within[8].

Reproduction

Starfish are capable of both sexual and asexual reproduction. Individual starfish are male or female. Fertilization takes place externally, both male and female releasing their gametes into the environment. Resulting fertilized embryos form part of the zooplankton.

Starfish are developmentally (embryologically) known as deuterostomes. Their embryo initially develops bilateral symmetry, indicating that starfish probably share a common ancestor with the chordates, which includes the fish.

Later development takes a very different path however as the developing starfish settles out of the zooplankton and develops the characteristic radial symmetry. Some species reproduce cooperatively, using environmental signals to coordinate the timing of gamete release; in other species, one to one pairing is the norm.

Some species of starfish also reproduce asexually by fragmentation, often with part of an arm becoming detached and eventually developing into an independent individual starfish. This has led to some notoriety.

Starfish can be pests to fishermen who make their living on the capture of clams and other mollusks at sea as starfish prey on these. The fishermen would presumably kill the starfish by chopping them up and disposing of them at sea, ultimately leading to their increased numbers until the issue was better understood. A starfish arm can only regenerate into a whole new organism if some of the central ring of the starfish is part of the chopped off arm.

Locomotion

Starfish move using a water vascular system. Water comes into the system via the madreporite. It is then circulated from the stone canal to the ring canal and into the radial canals. The radial canals carry water to the ampullae and provide suction to the tube feet. The tube feet latch on to surfaces and move in a wave, with one body section attaching to the surfaces as another releases.

Most starfish cannot move quickly. However, some burrowing species like starfish from genus Astropecten and Luidia are quite capable of rapid, creeping motion - it "glides" across the ocean floor. This motion results from their pointed tubefeet adapted specially for excavating local area of sand.

Regeneration

Some species of starfish have the ability to regenerate lost arms and can regrow an entire new arm in time. Most species must have the central part of the body intact to be able to regenerate, but a few can grow an entire starfish from a single ray. Included in this group are the red and blue Linckia star. The regeneration of these stars is possible due to the vital organs kept in their arms.

Habitat

Clownfish are native to wide ranges of the warm waters of

the Pacific and Indian Oceans; some species ranges overlap others. Clownfish are not found in the Atlantic Ocean. Clownfish live in a mutual relationship with sea anemones, or in some cases settle in some varieties of soft corals, or large polyp stony corals. Once an anemone or coral has been adopted, the clownfish will defend it.

The anemone is required in nature because reef life is dangerous for small, brightly coloured fish; in an aquarium lacking predators it is not needed. For this reason, clownfish never stray far from their host. In an aquarium, where they don't have to forage for food, it is very common for clownfish to remain within 6 to 12 inches of their host for an entire lifetime.

Clownfish and damselfish are the only species of fish which can avoid the potent stings of an anemone. There are several theories for how this avoidance is accomplished. Firstly, the slime coating of the fish may be based on sugar rather than proteins so anemones fail to recognize the fish as food and do not fire their nematocysts, or sting organelles.

Secondly, the mucous coating may mimic the anemone's own coating, a theory that is bolstered by the fact that it takes several days for a clownfish to adapt to a new species of anemone. There is no adaptation period when a clownfish is moved to another anemone of the same species.

Thirdly, their unique movements, which are unlike any other fish, may let the anemone know that they are not food. This theory is bolstered by the fact that juvenile clownfish, which have no coating, will immediately seek refuge in any compatible anemone and will not be stung. Juvenile clownfish will not survive for long without the protection of an anemone, and few find one before being eaten.

Clownfish live in their anemone in groups. Usually a female lives with other males. When the dominant female dies the head male changes sex and becomes the female.

Clownfish lay eggs on any flat surface close to or under protection of their host anemones. These eggs are cared for by the male and hatched under complete darkness after a period of 7 to 10 days. Hatching occurs in a natural rhythm directly connected to the phases of the moon. Clownfish are omnivorous, their diets range from flakes to meat. They feed mostly on copepods and mysids, the undigested excrement from their host anemones.

This coral occasionnaly has a fluorescent red or orange color during daytime; it has been found recently that this color is due to phycoerythrin, a cyanobacterial protein. It appears that, in addition to symbiotic zooxanthella, this

coral harbors endocellular symbiotic cyanobacteria, possibly to help it fix nitrogen.

Seahorses range in size from 16 mm (the recently discovered Hippocampus denise) to 35 cm. Seahorses and pipefishes are notable for being the only species in which males become "pregnant".

The seahorse has a dorsal fin located on the lower body and pectoral fins located on the head near their gills. Some species of seahorse are partly transparent and are rarely seen in pictures.

Sea dragons are close relatives of seahorses but have bigger bodies and leaf-like appendages which enable them

to hide among floating seaweed or kelp beds. Seahorses and sea dragons feed on larval fishes and amphipods, such as small shrimp-like crustaceans called mysids ("sea lice"), sucking up their prey with their small mouths. Many of these amphipods feed on red algae that thrives in the shade of the kelp forests where the sea dragons live.

Seahorses reproduce in an unusual way: the male becomes pregnant. "The female inserts her ovipositor into the male’s brood pouch, where she deposits her eggs, which the male fertilizes. The fertilized eggs then embed in the pouch wall and become enveloped with tissues."[4] New research indicates the male releases sperm into the surrounding sea water during fertilization, and not directly into the pouch as was previously thought.[5] Most seahorse species' pregnancies lasts approximately two to three weeks.

Hatched offspring are independent of their parents. Some spend time developing among the ocean plankton. At times, the male seahorse may try to consume some of the previously released offspring. Other species (H. zosterae) immediately begin life as sea-floor inhabitants (benthos).

Seahorses are generally monogamous, though several species (H. abdominalis among them) are highly gregarious. In monogamous pairs, the male and female will greet one another with courtship displays in the morning and sometimes in the evening to reinforce their pair bond. They spend the rest of the day separate from each other hunting for food.

Pets

While many aquarium hobbyists will keep seahorses as pets, seahorses collected from the wild tend to fare poorly in a home aquarium. They will eat only live foods such as brine shrimp and are prone to stress in an aquarium, which lowers the efficiency of their immune systems and makes them susceptible to disease.

In recent years, however, captive breeding of seahorses has become increasingly widespread. These seahorses survive better in captivity, and they are less likely to carry diseases. These seahorses will eat mysid shrimp, and they do not experience the shock and stress of being taken out of the wild and placed in a small aquarium. Although captive-bred seahorses are more expensive, they survive better than wild seahorses, and take no toll on wild populations.

Seahorses should be kept in an aquarium to themselves, or with compatible tank-mates. Seahorses are slow feeders, and in an aquarium with fast, aggressive feeders, the seahorses will be edged out in the competition for food. Special care should be given to ensure that all individuals obtain enough food at feeding times.

Seahorses can co-exist with many species of shrimp and other bottom-feeding creatures. Fish from the goby family also make good tank-mates. Some species are especially dangerous to the slow-moving seahorses and should be avoided completely: eels, tangs, triggerfish, squid, octopus, and sea anemones.

Animals sold as "freshwater seahorses" are usually the closely related pipefish, of which a few species live in the lower reaches of rivers. The supposed true "freshwater seahorse" called Hippocampus aimei was not a real species, but a name sometimes used for individuals of Barbour's seahorse and Hedgehog seahorse. The latter is a species commonly found in brackish waters, but not actually a freshwater fish.

Adaptations

A seahorse has highly mobile eyes to watch for predators and prey without moving its body. Like the leafy sea dragon, it also has a long snout with which it sucks up its prey. Its fins are small because it must move through thick water vegetation. The seahorse has a long, prehensile tail which it will curl around any support such as seaweed to prevent being swept away by currents.

The manta ray, or giant manta (Manta birostris), is the largest of the rays, with the largest known specimen having been nearly 7.6 meters (25 ft) across its pectoral fins (or "wings") and weighed in at 3,000 kg (6,600 lb). It ranges throughout the tropical seas of the world, typically around coral reefs.

Mantas are most commonly black above and white below, but some are blue on their backs. A giant manta's eyes are located at the base of the cephalic lobes on each side of the head, and unlike other rays the mouth is found at the anterior edge of its head. To breathe, like other rays, the manta has five pairs of gills on the underside.

With distinctive "horns" (from which the common name 'devil ray' stems), on either side of its broad head, the manta is a prized sighting by divers. These unique structures are actually derived from the pectoral fins.

During embryonic development, part of the pectoral fin breaks away and moves forward, surrounding the mouth. This gives the Manta Ray the distinction of being the only jawed vertebrate to have novel limbs (the so-called six-footed tortoise (Manouria emys) does not actually have six legs, only enlarged tuberculate scales on their thighs that look superficially like an extra pair of hind limbs).

These flexible horns are used to direct plankton, small fish and water into the Manta's very broad and wide mouth. To make them more streamlined when swimming, they are able to curl them up. Manta Rays may have evolved from bottom feeding ancestors but have adapted to become filter feeders in the open ocean. This has allowed them to grow to a larger size than any other species of ray.

Because of their pelagic lifestyle as plankton feeders, some of the ancestral characterstics have degenerated. For example, all that is left of their oral teeth is a small band of vestigial teeth on the lower jaw, almost hidden by the skin. Their dermal denticals are also greatly reduced in number and size, but are still present, and they have a much thicker body mucus coating than other rays. Their spiracles have become small and non-functional, as all water is taken in through their mouth instead.

To swim better through the ocean, they have a diamond shaped body plan, using their pectoral fins as graceful "wings".

Mantas generally eat plankton, fish larvae and small organisms that are filtered out from the water by their gill rakers, a type of filter feeding that is called ram-jet feeding.

The predators of the Manta Ray include mainly large sharks, however in some circumstances killer whales have also been observed preying on them.

Taxonomically, the situation of the mantas is still under investigation. Three species have been identified: Manta birostris, Manta ehrenbergii, and Manta raya, but they are quite similar to each other, and the last two may just be isolated populations. The genus Manta is sometimes placed in its own family, Mobulidae, but this article follows FishBase, and places it in the family Myliobatidae, with the eagle rays and their relatives.

Mantas have been given a variety of common names, including Atlantic manta, Pacific manta, devil ray, devilfish, and just manta. Some people just call all members of the family stingrays. Mantas also seem to recognize certain areas in coral reefs that harbor a certain type of fish. The mantas slow down and the fish swim inside the manta's gills to clean it of parasites.The manta rays are also have inspired the Bionicle character,Mantax.

The nine known species of hammerhead range from 2 to 6 m long (6.5 to 20 feet), and all species have a projection on each side of the head that give it a resemblance to a flattened hammer. The shark's eyes and nostrils are at the tips of the extensions.

They are aggressive predators which eat fish, rays,

cephalopods, and crustaceans. They are found in warmer waters along coastlines and continental shelves.

The hammer shape of the head was once thought to act as a wing, aiding in close-quarters maneuverability and allowing the shark to execute sharp turns without loss of stability. However, it was found that the special design of its vertebra allowed it to make the turns correctly, more than its head. But as a wing the hammer would also provide lift; hammerheads are one of the most negatively buoyant of sharks.

Like all sharks, hammerheads have electrolocation sensory pores called ampullae of Lorenzini. By distributing the receptors over a wider area, hammerheads can sweep for prey more effectively[1]. These sharks have been able to detect an electrical signal of half a billionth of a volt. The hammer shaped head also gives these sharks larger nasal tracts, increasing the chance of finding a particle in the water by at least 10 times as compared to other 'classical' sharks.

Wider spacing between sensory organs better enables an organism to detect gradients and therefore the location of a gradient source such as food or a mate. The peculiar head of this shark can be thought of as analogous to the antennae of an insect.

Hammerheads have proportionately small mouths and seem to do a lot of bottom-hunting. They are also known to form schools during the day, sometimes in groups of over 100. In the evening, like other sharks, they become solo hunters.

Reproduction

Reproduction in the hammerhead shark occurs once a year and each litter contains 20 to 40 pups. Hammerhead shark mating courtship is a very violent affair. The male will bite the female until she acquiesces, allowing mating to occur. Unlike many other shark species, the hammerhead shark has internal fertilization which creates a safe environment for the sperm to unite with the egg.

The embryo develops within the female inside a placenta and is fed through an umbilical cord, similar to mammals. The gestation period is 10 to 12 months. Once the pups are born the parents do not stay with them and they are left to fend for themselves. A world-record 1,280 pound (580 kg) pregnant female hammerhead shark was caught off Boca Grande, Florida on May 23, 2006. The shark was carrying 55 pups, which suggests scientists had previously underestimated the number of pups per gestation.

Dolphins are aquatic mammals which are closely related to whales and porpoises. There are almost forty species of dolphin in seventeen genera. They vary in size from 1.2 metres (4 ft) and 40 kilograms (88 lb) (Maui's Dolphin), up to 9.5 m (30 ft) and ten tonnes (the Orca).

They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating fish and squid.

The family Delphinidae is the largest in the Cetacea, and relatively recent: dolphins evolved about ten million years ago, during the Miocene. Dolphins are considered to be amongst the most intelligent of animals and their often

friendly appearance and seemingly playful attitude have made them popular in human culture.

Origin of the name

The name is from Ancient Greek de?f?? delphis meaning "with a womb" which can be interpreted as meaning "a 'fish' with a womb".

The word is used in a few different ways. It can mean:

Any member of the family Delphinidae (oceanic dolphins),

Any member of the families Delphinidae and Platanistoidea (oceanic and river dolphins),

Any member of the suborder Odontoceti (toothed whales; these include the above families and some others),

Used casually as a synonym for Bottlenose Dolphin, the most common and familiar species of dolphin.

In this article, the second definition is used. Porpoises (suborder Odontoceti, family Phocoenidae) are thus not dolphins in this sense. Orcas and some closely related species belong to the Delphinidae family and therefore qualify as dolphins, even though they are called whales in common language. A group of dolphins can be called a "school" or a "pod".

Evolution

Dolphins, along with whales and porpoises, are thought to be descendants of terrestrial mammals, most likely of the Artiodactyl order. The ancestors of the modern day dolphins entered the water roughly fifty million years ago. Modern dolphin skeletons have two small, rod-shaped pelvic bones thought to be vestigial hind legs. In October 2006 an unusual Bottlenose Dolphin was captured in Japan; it had small fins on each side of its genital slit which scientists believe to be a more pronounced development of these vestigial hind legs.

Anatomy

Dolphins have a streamlined fusiform body, adapted for fast swimming. The basic colouration patterns are shades of grey with a light underside and a distinct dark cape on the back. It is often combined with lines and patches of different hue and contrast.

The head contains the melon, a round organ used for echolocation. In many species, the jaws are elongated, forming a distinct beak; for some species like the Bottlenose, there is a curved mouth which looks like a fixed smile. Teeth can be very numerous (up to two hundred and fifty) in several species. The dolphin brain is large and has a highly structured cortex, which often is referred to in discussions about their advanced intelligence.

Unlike most mammals, dolphins do not have hair, but they are born with a few hairs around the tip of their rostrum which they lose after some time, in some cases even before they are born. The only exception to this is the Boto river dolphin, which does have some small hairs on the rostrum.

Their reproductive organs are located on the underside of the body. Males have two slits, one concealing the penis and one further behind for the anus. The female has one genital slit, housing the vagina and the anus. A mammary slit is positioned on either side of the female's genital slit.

Senses

Most dolphins have acute eyesight, both in and out of the water, and their sense of hearing is superior to that of humans. Though they have a small ear opening on each side of their head, it is believed that hearing underwater is also if not exclusively done with the lower jaw which conducts the sound vibrations to the middle ear via a fat-filled cavity in the lower jaw bone.

Hearing is also used for echolocation, which seems to be an ability all dolphins have. Their teeth are arranged in a way that works as an array or antenna to receive the incoming sound and make it easier for them to pinpoint the exact location of an object. The dolphin's sense of touch is also well-developed.

However, dolphins lack an olfactory nerve and lobes and thus are believed to have no sense of smell, but they can taste and do show preferences for certain kinds of fish. Since dolphins spend most of their time below the surface normally, just tasting the water could act in a manner analogous to a sense of smell.

Though most dolphins do not have any hair, they do still have hair follicles and it is believed these might still perform some sensory function, though it is unclear what exactly this may be. The small hairs on the rostrum of the Boto river dolphin are believed to function as a tacticle sense however, possibly to compensate for the Boto's poor eyesight.

Reproduction

Dolphin copulation happens belly to belly and though many species engage in lengthy foreplay, the actual act is usually only brief, but may be repeated several times with a few minutes in between. The gestation period varies per species.

Dolphins are one of the few animals other than humans known to mate for reasons other than reproduction. Male Bottlenose Dolphins are known to engage in sexual acts with other dolphin species, which is not always consensual, though the Bottlenose may also be submissive in such encounters. Occasionally, dolphins will also show sexual behaviour towards humans.

Sea turtles (Chelonioidea) are turtles found in all the world's oceans except the Arctic Ocean, and some species travel between oceans. The Flatback turtle is found solely on the northern coast of Australia.

The Leatherback Sea Turtle is the largest, measuring six or seven feet (2 m) in length at maturity, and three to five feet (1 to 1.5 m) in width, weighing up to 1300 pounds (600 kg). Most other species are smaller, being two to four feet in length (0.5 to 1 m) and proportionally less wide.

There are seven types of sea turtles: Kemp's Ridley,

Flatback, Green, Olive Ridley, Loggerhead, Hawksbill and the Leatherback. All but the Leatherback are in the family Chelonioidea; the Leatherback belongs to the family Dermochelyidae and is its only member.

Different species are distinguished by varying anatomical aspects: for instance, the prefrontal scales on the head, the number of and shape of scutes on the carapace, and the type of inframarginal scutes on the plastron. The Leatherback is the only sea turtle that does not have a hard shell, instead carrying a mosaic of bony plates beneath its leathery skin.

Life History

Sea turtles have an extraordinary sense of time and location. They are highly sensitive to the Earth's magnetic field and use it to navigate. They can live up to 189 years. The fact that most species return to nest at the locations where they were born seems to indicate an imprint of that location's magnetic features.

The Ridley turtles are especially peculiar because instead of nesting individually like the other species, they come ashore in one mass arrival known as an "arribada" (arrival). With the Kemp's Ridley this occurs during the day and on only one beach in the entire world. Their numbers used to range in the thousands but due to the effects of extensive egg poaching and hunting in previous years the numbers are now in the hundreds.

After about 30 years of maturing, adult female sea turtles return to the land to nest at night, usually on the same beach from which they hatched. This can take place every two to four years in maturity. They make from four to seven nests per nesting season. They dig a hole with their hind flippers and lay from 70 to 190 eggs in it (depending on the species) before covering it up and returning to the ocean.

Some of the eggs are unfertilized 'dummy eggs' and the rest contain young turtles. Incubation takes about 2 months. The length of incubation and the gender of the hatchling depends on the temperature of the sand. Darker sands maintain higher temperatures, decreasing incubation time and increasing the frequency of female hatchlings.

When the eggs hatch, these baby turtles dig their way out and seek the ocean. Only a very small proportion of them (usually .001%) will be successful, as many predators wait to eat the steady stream of new hatched turtles (since many sea turtles lay eggs en masse, the eggs also hatch en masse).

The hatchlings then proceed into the open ocean, borne on oceanic currents that they often have no control over. While in the open ocean, it used to be the case that what happened to sea turtle young during this stage in their lives was unknown.

However in 1987, it was discovered that the young of Chelonia mydas and Caretta caretta spent a great deal of their pelagic lives in floating sargassum beds - thick mats of unanchored seaweed floating in the middle of the ocean. Within these beds, they found ample shelter and food. In the absence of sargassum beds, turtle young feed in the vicinity of upwelling "fronts"

Sea turtles and fragile ecosystems

Sea turtles play key roles in two ecosystems that are critical to them as well as to humans—the oceans and beaches/dunes. If sea turtles were to become extinct, the negative impact on beaches and the oceans would potentially be significant.

In the oceans, for example, sea turtles, especially green sea turtles, are one of the very few creatures (manatees are another) that eat a type of vegetation called sea grass that grows on the sea floor. Sea grass must be kept short to remain healthy, and beds of healthy sea grass are essential breeding and development areas for many species of fish and other marine life.

A decline or loss of sea grass beds would mean a loss of the marine species that directly depend on the beds, which would trigger a chain reaction and negatively impact marine and human life. When one part of an ecosystem is destroyed, the other parts will follow.

Beaches and dunes are a fragile ecosystem that does not get many nutrients to support its vegetation, which is needed to help prevent erosion. Sea turtles contribute nutrients to dune vegetation from their eggs. Every year, sea turtles lay countless numbers of eggs in beaches during nesting season.

Along one twenty-mile stretch of beach in Florida alone, for example, more than 150,000 pounds of eggs are laid each year. Nutrients from hatched eggs as well as from eggs that never hatch and from hatchlings that fail to make it into the ocean are all sources of nutrients for dune vegetation. A decline in the number of sea turtles means fewer eggs laid, less nutrients for the sand dunes and its vegetation, and a higher risk for beach erosion.

The plight of sea turtles has been recognized around the world, and many organizations and governments are working to preserve these ancient creatures. Volunteer opportunities to save the turtles are available in North America and around the world.

Jellyfish are marine invertebrates belonging to the Scyphozoan class. The body of an adult jellyfish is composed of a bell-shaped, jelly producing substance enclosing its internal structure, from which the creature's tentacles are suspended.

Each tentacle is covered with stinging cells (cnidocytes) that can sting or kill other animals: most jellyfish use them to secure prey or as a defense mechanism. Others, such as Rhizostomae, do not have tentacles at all.

To compensate for its lack of basic sensory organs and a brain, the jellyfish exploits its nervous system and rhopalia to perceive stimuli, such as light or odor, and orchestrate expedient responses.

In its adult form, it is composed of 94–98% water and can

be found in every ocean in the world. Most jellyfish are passive drifters that feed on small fish and zooplankton that become caught in their tentacles. Jellyfish have an incomplete digestive system, meaning that the same orifice is used for both food intake and waste expulsion.

They are made up of a layer of epidermis, gastrodermis, and a thick layer called mesoglea that produces most of the jelly and separates the epidermis from the gastrodermis.

Their shape is not hydrodynamic, which makes them slow swimmers but this is little hindrance as they feed on plankton, needing only to drift slowly through the water. It is more important for them that their movements create a current where the water (which contains their food) is being forced within reach of their tentacles. They accomplish this by rhythmically opening and closing their bell-like body.

Since jellyfish do not biologically qualify as actual "fish", the term "jellyfish" is considered a misnomer by some, who instead employ the names "jellies" or "sea jellies". The name "jellyfish" is also often used to denote either Hydrozoa or the box jellyfish, Cubozoa. The class name Scyphozoa comes from the Greek word skyphos, denoting a kind of drinking cup and alluding to the cup shape of the animal.

Life cycle and reproduction

Most jellyfish pass through two different body forms during their life cycle. The first is the polyp stage; in this phase, the jellyfish takes the form of either a sessile stalk which catches passing food, or a similar free-floating configuration. The polyp's mouth and tentacles are facing upwards.

In the second stage, the jellyfish is known as a medusa. Medusae have a radially symmetric, umbrella-shaped body called a bell. The medusa's tentacles hang from the border of the bell.

Jellyfish are dioecious (that is, they are either male or female). In most cases, to reproduce, a male releases his sperm into the surrounding water. The sperm then swims into the mouth of the female jelly, allowing the fertilization of the ova process to begin. Moon jellies, however, use a different process: their eggs become lodged in pits on the oral arms, which form a temporary brood chamber to accommodate fertilization.

After fertilization and initial growth, a larval form, called the planula, develops from the egg. The planula larva is small and covered with cilia. It settles onto a firm surface and develops into a polyp.

The polyp is cup-shaped with tentacles surrounding a single orifice, perhaps resembling a tiny sea anemone. Once the polyp begins reproducing asexually by budding, it's called a segmenting polyp, or a scyphistoma. New scyphistomae may be produced by budding or new, immature jellies called ephyra may be formed. Many jellyfish can bud off new medusae directly from the medusan stage.

Most jellyfish do not live longer than six months, two and a half months being more common.

Defense and feeding mechanisms

Most jellyfish have tentacles or oral arms coated with thousands of microscopic nematocysts; generally, each of these nematocyst has a "trigger" (cnidocil) paired with a capsule containing a coiled stinging filament, as well as barbs on the exterior. Upon contact, the filament will swiftly unwind, launch into the target, and inject toxins. It can then pull the victim into its mouth, if appropriate.

Although most jellyfish are not perniciously dangerous to humans, a few are highly toxic, such as Cyanea capillata. The recently discovered Carukia barnesi is also suspected of causing two deaths in Australia.

Contrary to popular belief, the menacingly infamous Portuguese Man o' War (Physalia) is not actually a jellyfish, but a colony of hydrozoan polyps. Regardless of the actual toxicity of the stings, many victims find them very painful, and some individuals may have severe allergic reactions similar to bee sting allergic reactions.

Body systems

A jellyfish can detect the touch of other animals using a nervous system called a "nerve net", which is found in its epidermis. Impulses to the nerve cells are sent from nerve rings that have collected information from the environment of the jellyfish through the rhopalial lappet, which is located around the animal's body.

Jellyfish also have ocelli that cannot form images, but are sensitive to light; the jellyfish can use these to determine up from down, basing its judgement on sunlight shining on the surface of the water.

Jellyfish do not have a specialized digestive system, osmoregulatory system, central nervous system, respiratory system, or circulatory system. They are able to digest with the help of the gastrodermis that lines the gastrovascular cavity, where nutrients from their food are absorbed.

They do not need a respiratory system since their skin is thin enough that oxygen can easily diffuse in and out of their bodies. Jellyfish have limited control over their movement and mostly free-float, but can use a hydrostatic skeleton that controls the water pouch in their body to actuate vertical movement.

In cell biology, ectoplasm ("outer plasma") refers to the outer regions of jelly fish. The jelly like material called (ectoplasma or plassy for short) typically contains a smaller amount of protein granules and other organic compounds than inner cytoplasm, also referred to as endoplasm.

Blooms and grouping

A group of jellyfish is often called a "smack."[citation needed] and in Australian slang a group of jellyfish is known as a "slut".[citation needed] Many species of jellyfish are also capable of congregating into large swarms or "blooms" consisting of hundreds or even thousands of individuals.

The formation of these blooms is a complex process that depends on ocean currents, nutrients, temperature and oxygen content. Jellyfish will sometimes mass breed during blooms. Jellyfish population is reportedly raising major ecological concerns for a possible jellyfish outbreak.

According to Claudia Mills of the University of Washington, the frequency of these blooms may be attributed to mankind's impact on marine life. She says that the breeding jellyfish may merely be taking the place of already overfished creatures.

Jellyfish researcher Marsh Youngbluth further clarifies that "jellyfish feed on the same kinds of prey as adult and young fishes, so if fish are removed from the equation, jellyfish are likely to move in."

Increased nutrients in the water, ascribed to agricultural runoff, have also been cited as an antecedent to the recent proliferation of jellyfish numbers. Scientist Monty Graham says that "ecosystems in which there are high levels of nutrients ... provide nourishment for the small organisms on which jellyfish feed.

In waters where there is eutrophication, low oxygen levels often result, favoring jellyfish as they thrive in less oxygen-rich water than fish can tolerate. The fact that jellyfish are increasing is a symptom of something happening in the ecosystem."

By sampling sea life in a heavily fished region off the coast of Namibia, researchers have found that jellyfish have actually overtaken fish in terms of the biomass they contribute to this ocean region.

The findings represent a careful quantitative analysis of what has been called a "jellyfish explosion" following intense fishing in the area in the last few decades. The findings were reported by Andrew Brierley of the University of St. Andrews and his colleagues in the July 12, 2006 issue of the journal Current Biology.

Areas seriously affected by jellyfish blooms include the northern Gulf of Mexico, where "moon jellies have formed a kind of gelatinous net that stretches from end to end across the gulf," and the Adriatic Sea. Some jellyfish have even been spotted along coastal shores.

Captivity

Jellyfish are commonly displayed in aquariums across the United States and in other countries; among the more known are the Monterey Bay Aquarium, Long Beach Aquarium of the Pacific, Vancouver Aquarium, and Maui Ocean Center. Often the tank's background is blue with the animals illuminated by side lighting to produce a high contrast effect. In natural conditions, many of the jellies are so transparent that they can be almost impossible to see.

Holding jellies in captivity also presents other problems: for one, they are not adapted to closed spaces or areas with walls, which aquariums by definition have. They also depend on the natural currents of the ocean to transport them from place to place.

To compensate for this, professional exhibits feature precise water flow patterns, typically in circular tanks to prevent specimens from becoming trapped in corners. The Monterey Bay Aquarium uses a modified version of the "kreisel" (German for "spinning top") for this purpose.

Treatment of stings

When stung by a jellyfish, first aid may be in order. Though most jellyfish stings are not deadly, some stings, such as those perpetrated by the box jellyfish (Chironex fleckeri) may be fatal. Serious stings may cause anaphylaxis and eventual death, and hence people stung by jellyfish must get out of the water to avoid drowning. In these serious cases, advanced professional care must be sought.

This care may include administration of an antivenom and other supportive care such as required to treat the symptoms of anaphylactic shock. The most serious threat that humans face from jellyfish is the sting of the Irukandji, which has the most potent and deadly poison of any known jellyfish species.

There are three goals of first aid for uncomplicated jellyfish stings: prevent injury to rescuers, inactivate the nematocysts, and remove any tentacles stuck on the patient. To prevent injury to rescuers, barrier clothing should be worn. This protection may include anything from panty hose to wet suits to full-body sting-proof suits. Inactivating the nematocysts, or stinging cells, prevents further injection of venom into the patient.

Vinegar (3 to 10% acetic acid in water) should be applied for box jellyfish stings. However, vinegar is not recommended for Portuguese Man o' War stings. In the case of stings on or around the eyes, vinegar may be placed on a towel and dabbed around the eyes, but not in them.

Salt water may also be used in case vinegar is not readily available. Fresh water should not be used if the sting occurred in salt water, as a change in pH can cause the release of additional venom. Rubbing the wound, or using alcohol, spirits, ammonia, or urine will encourage the release of venom and should be avoided.

Once deactivated, the stinging cells must be removed. This can be accomplished by picking off tentacles left on the body First aid providers should be careful to use gloves or another readily available barrier device to prevent personal injury, and to follow standard universal precautions.

After large pieces of the jellyfish are removed, shaving cream may be applied to the area and a knife edge, safety razor, or credit card may be used to take away any remaining nematocysts.

Beyond initial first aid, antihistamines such as diphenhydramine (Benadryl) may be used to control skin irritation (pruritus).

Taxonomy

Seaweeds are often confused with other photosynthetic organisms. Seaweeds are popularly described as plants, but biologists typically do not consider them true Plantae. They should not be confused either with plants, such as seagrasses (which are vascular plants). In addition, a few species of cyanobacteria bear a resemblance to seaweed algae. Many phycologists prefer the term "marine macroalgae" over "seaweeds".

History

In the early 19th century seaweeds were treated with disdain by some:

There was a time when a student who, having collected some beautiful algae on the shore, showed the contents of his vasculum to the Professor of Botany, expressing a wish to get some information respecting them. The Professor looked at them, and putting on his spectacles, again looked at them, when, pushing them from him, he exclaimed: "Pooh! a parcel of Seaweeds, Sir; a parcel of Seaweeds!"

Structure

Seaweeds' appearance somewhat resembles non-arboreal terrestrial plants.

thallus: the algal body

lamina: a flattened structure that is somewhat leaf-like

sorus: spore cluster

on Fucus, air bladders: float-assist organ (on blade)

on kelp, floats: float-assist organ (between lamina and stipe)

stipe: a stem-like structure, may be absent

holdfast: specialized basal structure providing attachment to a surface, often a rock or another alga.

The stipe and blade are collectively known as fronds.

Ecology

The ecology of seaweeds is dominated by two specific environmental requirements. These are the presence of sea-water (or at least brackish water) and the presence of light sufficient to drive photosynthesis. A very common requirement is also to have a firm point of attachment. As a result, seaweeds are most commonly found in the littoral zone and within that zone more frequently on rocky shores than on sand or shingle. The ecological niches utilised by seaweeds are wide ranging.

At the highest level are those that inhabit the zone that is only wetted by sea spray through top the deepest living that are attached to the sea-bed under several metres of water. In some parts of the world, the area colonized by littoral seaweeds can extend for several miles away from the shore. The limiting factor in such cases is the availability of sufficient sun-light to support photosynthesis. The deepest living sea-weeds are the various kelps.

In addition to the familiar sea-shore seaweeds, a number of species have adapted to a fully planktonic niche and are free-floating, often with the assistance of gas filled sacs. Sargassum is one of the better know examples of this type of seaweed.

A number of species have adapted to the specialised environment of tidal rock pools. In this niche seaweeds are able to withstand rapidly changing temperature and salinity and even occasional drying.

The red algae (Rhodophyta, IPA: [?r??d?(?)'f??t?], from Greek: ??d?? (rhodon) = rose + f?t?? (phyton) = plant, thus red plant) are a large group, about 5000 - 6000 species of mostly multicellular, marine algae, including many notable seaweeds.

Other references indicate 10,000 species. Most of the coralline algae, which secrete calcium carbonate and play a major role in building coral reefs, belong here. Red algae

such as dulse and nori are a traditional part of European and Asian cuisine and are used to make other products like agar, carrageenans and other food additives

Fossil record

The oldest fossil identified as a red alga is also the oldest fossil eukaryote that belongs to a specific modern taxon. Bangiomorpha pubescens, a multicellular fossil from arctic Canada, strongly resembles the modern red alga Bangia despite occurring in rocks dating to 1200 million years ago.

Red algae are important builders of limestone reefs. The earliest such coralline algae, the solenopores, are known from the Cambrian Period. Other algae of different origins filled a similar role in the late Paleozoic, and in more recent reefs.

Species

There are around 10,100 known species, with nearly all of them [marine,http://www.seaweed.ie/algae/rhodophyta.lasso] and only 200 that live in freshwater. Some examples are:

Atractophora hypnoides

Gelidiella calcicola

Lemanea

Palmaria palmata

Schmitzia hiscockiana

Chondrus crispus

Mastocarpus stellatus

Pit connections and pit plugs

How pit connections are formed

Pit connections and pit plugs are unique and distinctive features of red algae that form during the process of cytokinesis following mitosis. In red algae, cytokinesis is incomplete. Typically, a small pore is left in the middle of the newly formed partition. The pit connection is formed where the daughter cells remain in contact.

Shortly after the pit connection is formed cytoplasmic continuity is blocked by the generation of a pit plug, which is deposited in the wall gap that connects the cells.

Connections between cells having a common parent cell are called a primary pit connections. Because apical growth is the norm in red algae, most cells have two primary pit connections, one to each adjacent cell.

Connections that exist between cells not sharing a common parent cells are labeled secondary pit connections. These connections are formed when an unequal cell division produced a nucleated daughter cell that then fuses to an adjacent cell. Patterns of secondary pit connections can be seen in the order Ceramiales.

How pit plugs are formed

After a pit connection is formed, tubular membranes appear. A granular protein, called the plug core, then forms around the membranes. The tubular membranes eventually disappearWhile some orders of red algae simply have a plug core, others have an associated membrane at each side of the protein mass, called cap membranes. The pit plug continues to exist between the cells until one of the cells dies. When this happens, the living cell produce a layer of wall material that seals off the plug.

Function

It is thought that the pit connections function as structural reinforcement, and as an avenue for cell to cell communication and/or symplastic transport in red algae. While the presence of the cap membrane could inhibit this transport between cells, it has been hypothesized that the tubular plug cores serve as a means of transport.

Algae are photosynthetic organisms that occur in most habitats. Algae varies from small, single-celled species to complex multicellular species, such as the giant kelps that grow to 65 meters in length.

Although algae have conventionally been regarded as

simple plants, they actually span more than one domain, including both Eukaryota and Bacteria (see Blue-green algae), as well as more than one kingdom, including plants and protists, the latter being traditionally considered more animal-like (see Protozoa). Thus algae do not represent a single evolutionary direction or line but a level of organization that may have developed several times in the early history of life on Earth.

Algae range from single-cell organisms to multicellular organisms, some with fairly complex differentiated form and (if marine) called seaweeds. All lack leaves, roots, flowers, seeds and other organ structures that characterize higher plants (vascular plants).

They are distinguished from other protozoa in that they are photoautotrophic although this is not a hard and fast distinction as some groups contain members that are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy. Some unicellular species rely entirely on external energy sources and have reduced or lost their photosynthetic apparatus.

All algae have photosynthetic machinery ultimately derived from the cyanobacteria, and so produce oxygen as a byproduct of photosynthesis, unlike non-cyanobacterial photosynthetic bacteria. It is estimated that algae produce about 73 to 87 percent of the net global production of oxygen - which is available to humans and other terrestrial animals for respiration.

Algae are usually found in damp places or bodies of water and thus are common in terrestrial as well as aquatic environments. However, terrestrial algae are usually rather inconspicuous and far more common in moist, tropical regions than dry ones, because algae lack vascular tissues and other adaptations to live on land. Algae can, however, endure dryness and other conditions in symbiosis with a fungus as lichen.

The various sorts of algae play significant roles in aquatic ecology. Microscopic forms that live suspended in the water column — called phytoplankton — provide the food base for most marine food chains. In very high densities (so-called algal blooms) these algae may discolor the water and outcompete or poison other life forms. Seaweeds grow mostly in shallow marine waters. Some are used as human food or harvested for useful substances such as agar or fertilizer.

The study of marine and freshwater algae is called phycology or algology.

The US Algal Collection is represented by almost 300,000 accessioned and inventoried herbarium specimens.

Modification from ancestral forms

Squid have differentiated from their ancestral molluscs in

such a way that the body plan has been condensed antero-posteriorly and extended dorso-ventrally. What before may have been the foot of the ancestor is now modified into a complex set of tentacles and highly developed sense organs, including advanced eyes similar to those of vertebrates.

The shell of the ancestor has been lost, with only an internal gladius, or pen, remaining.

Anatomy

The main body mass of the squid is enclosed in the mantle, which has two swimming fins along each side. It should be noted that these fins, unlike in other marine organisms, are not the main source of ambulation. The skin of the squid is covered in chromatophores, which enable the squid to change color to suit its surroundings. The underside of the squid is also found to be lighter than the topside, in order to provide camouflage from both prey and predator.

Under the body are openings to the mantle cavity, which contains the gills (ctenidia) and openings to the excretory and reproductive systems. At the front of the mantle cavity lies the siphon, which the squid uses for locomotion via precise jet propulsion.

This is done by sucking water into the mantle cavity and quickly expelling it out of the siphon in a fast, strong jet. The direction of the siphon can be changed in order to suit the direction of travel.

Inside the mantle cavity, beyond the siphon, lies the visceral mass of the squid, which is covered in a thin, membranous epidermis. Under this are all the major internal organs of the squid.

Reproductive system

In female squid, the ink sac is hidden from view by a pair of white nidamental glands, which lie anterior to the gills. There are also red-spotted accessory nidamental glands. Both of these organs are associated with manufacture of food supplies and shells for the eggs. Females also have a large translucent ovary, situated towards the posterior of the visceral mass.

Male squid do not possess these organs, but instead have a large testis in place of the ovary, and a spermatophoric gland and sac. In mature males, this sac may contain spermatophores, which are placed inside the mantle of the female during mating.

Digestive system

Squid, like all cephalopods, have complex digestive systems. Food is transported into a muscular stomach, found roughly midpoint in the visceral mass. The bolus is then transported into the caecum for digestion. The caecum, a long, white organ, is found next to ovary or testis.

In mature squid, more priority is given to reproduction and so the stomach and caecum often shrivel up towards the later stages of life. Finally, food goes to the liver (or digestive gland), found at the siphon end of the squid, for absorption. Solid waste is passed out of the rectum. Beside the rectum is the ink sac, which allows a squid to discharge a black ink into the mantle cavity at short notice.

Cardiovascular system

Squid have three hearts. Two branchial hearts, feeding the gills, each surrounding the larger systemic heart that pumps blood around the body. The hearts have a faint greenish appearance and are surrounded by the renal sacs - the main excretory system of the squid. The kidneys are faint and difficult to identify and stretch from the hearts (located at the posterior side of the ink sac) to the liver. The systemic heart is made of three chambers, a lower ventricle and two upper auricles.

Head

The head end of the squid bears 8 arms and two tentacles, each a form of muscular hydrostat containing many suckers along the edge. These tentacles do not grow back if severed. In mature male squid, one basal half of the left ventral tentacle is hectocotylised - it ends in a copulatory pad rather than suckers. This is used for intercourse between mature males and females.

The mouth of the squid is equipped with a sharp horny beak mainly made of chitin and cross-linked proteins, and is used to kill and tear prey into manageable pieces. In contrast to the teeth and jaws of many other organisms, including from marine species, the beak does not contain any minerals.

Yet it is a very robust structure. Captured whales often have squid beaks in their stomachs, the beak being the only indigestible part of the squid. The mouth contains the radula (the rough tongue common to all molluscs except bivalvia and aplacophora).

The eyes of the squid, found on either side of the head, contains a hard lens, which is used much like the lens of a camera or telescope for focusing; rather than changing shape, like a human eye, it moves mechanically.

Physiology

Octopuses are characterized by their eight arms (not

tentacles), usually bearing suction cups. These arms are a type of muscular hydrostat. Unlike most other cephalopods, the majority of octopuses — those in the suborder most commonly known, Incirrina — have almost entirely soft bodies with no internal skeleton. They have neither a protective outer shell like the nautilus, nor any vestige of an internal shell or bones, like cuttlefish or squids.

A beak, similar in shape to a parrot's beak, is the only hard part of their body. This enables them to squeeze through very narrow slits between underwater rocks, which is very helpful when they are fleeing from morays or other predatory fish. The octopuses in the less familiar Cirrina suborder have two fins and an internal shell, generally lessening their ability to squeeze into small spaces.

Octopuses have a relatively short life span, and some species live for as little as six months. Larger species, such as the North Pacific Giant Octopus, may live for up to five years under suitable circumstances. However, reproduction is a cause of death: males can only live for a few months after mating, and females die shortly after their eggs hatch, for they neglect to eat during the (roughly) one month period spent taking care of their unhatched eggs.

Octopuses have three hearts. Two pump blood through each of the two gills, while the third pumps blood through the body. Octopus blood contains the copper-rich protein hemocyanin for transporting oxygen. Less efficient than the iron-rich hemoglobin of vertebrates, the hemocyanin is dissolved in the plasma instead of being bound in red blood cells and gives the blood a blue color. Octopuses draw water into their mantle cavity where it passes through its gills. As mollusks, octopuses have gills that are finely divided and vascularized outgrowths of either the outer or the inner body surface.

Intelligence

Octopuses are highly intelligent, probably more intelligent than any other order of invertebrates. The exact extent of their intelligence and learning capability is much debated among biologists, but maze and problem-solving experiments have shown that they do have both short- and long-term memory. Their short lifespans limit the amount they can ultimately learn.

There has been much speculation to the effect that almost all octopus behaviors are independently learned rather than instinct-based, although this remains largely unproven. They learn almost no behaviors from their parents, with whom young octopuses have very little contact.

An octopus has a highly complex nervous system, only part of which is localized in its brain. Two-thirds of an octopus's neurons are found in the nerve cords of its arms, which have a remarkable amount of autonomy. Octopus arms show a wide variety of complex reflex actions arising on at least three different levels of the nervous system. Some octopuses, such as the Mimic Octopus, will move their arms in ways that emulate the movements of other sea creatures.

In laboratory experiments, octopuses can be readily trained to distinguish between different shapes and patterns. They have been reported to practice observational learning, although the validity of these findings is widely contested on a number of grounds. Octopuses have also been observed in what some have described as play: repeatedly releasing bottles or toys into a circular current in their aquariums and then catching them. Octopuses often break out of their aquariums and sometimes into others in search of food. They have even boarded fishing boats and opened holds to eat crabs.

In some countries octopuses are on the list of experimental animals on which surgery may not be performed without anesthesia. In the UK, cephalopods such as octopuses are regarded as honorary vertebrates under the Animals (Scientific Procedures) Act 1986 and other cruelty to animals legislation, extending to them protections not normally afforded to invertebrates.

A common belief is that when stressed, an octopus may begin to eat its own arms. However, limited research conducted in this area has revealed that the cause of this abnormal behavior, called autophagy, may be a virus that attacks the octopus's nervous system. Thus this behavior may be more correctly labeled as a neurological disorder.

Defense

Three defensive mechanisms are typical of octopuses: ink sacs, camouflage, and autotomising limbs.

Most octopuses can eject a thick blackish ink in a large cloud to aid in escaping from predators. The main colouring agent of the ink is melanin, which is the same chemical that gives humans their hair and skin colour. This ink cloud dulls smell, which is particularly useful for evading predators that are dependent on smell for hunting, such as sharks.

An octopus's camouflage is aided by certain specialized skin cells which can change the apparent color, opacity, and reflectiveness of the epidermis. Chromatophores contain yellow, orange, red, brown, or black pigments; most species have three of these colors, while some have two or four. Other color-changing cells are reflective iridophores, and leucophores (white). This color-changing ability can also be used to communicate with or warn other octopuses. The very venomous blue-ringed octopus becomes bright yellow with blue rings when it is provoked.

When under attack, some octopuses can detach their own limbs, in a similar manner to the way skinks and other lizards detach their tails. The crawling arm serves as a distraction to would-be predators; this ability is also used in mating.

A few species, such as the Mimic Octopus, have a fourth defense mechanism. They can combine their highly flexible bodies with their color changing ability to accurately mimic other, more dangerous animals such as lionfish, sea snakes and eels. They have also been observed changing the texture of their mantle in order to achieve a greater camouflage. The mantle can take on the spiky appearance of seaweed, or the scraggly, bumpy texture of a rock, among other disguises.

Reproduction

When octopuses reproduce, males use a specialized arm called a hectocotylus to insert spermatophores (packets of sperm) into the female's mantle cavity. The hectocotylus, usually the third right arm, detaches from the male during copulation. Males die within a few months after mating. In some species, the female octopus can keep the sperm alive inside her for weeks until her eggs are mature.

After they have been fertilized, the female lays about 200,000 eggs (this figure dramatically varies between families, genera, species and also individuals). The female hangs these eggs in strings from the ceiling of her lair, or individually attaches them to the substratum depending on the species. The female cares for the eggs, guarding them against predators, and gently blowing currents of water over them so that they get enough oxygen. The female does not eat during the roughly one-month period spent taking care of the unhatched eggs.

At around the time the eggs hatch, the mother dies and the young larval octopuses spend a period of time drifting in clouds of plankton, where they feed on copepods, larval crabs and larval starfish until they are ready to sink down to the bottom of the ocean, where the cycle repeats itself. In some deeper dwelling species, the young do not go through this period. This is a dangerous time for the larval octopuses; as they become part of the plankton cloud they are vulnerable to many plankton eaters.

Sensation

Octopuses have keen eyesight. Although their slit-shaped pupils might be expected to afflict them with astigmatism, it appears that this is not a problem in the light levels in which an octopus typically hunts. Surprisingly, they do not appear to have color vision, although they can distinguish the polarization of light. Attached to the brain are two special organs, called statocysts, that allow the octopus to sense the orientation of its body relative to horizontal. An autonomic response keeps the octopus's eyes oriented so that the pupil slit is always horizontal.

Octopuses also have an excellent sense of touch. An octopus's suction cups are equipped with chemoreceptors so that the octopus can taste what it is touching. The arms contain tension sensors so that the octopus knows whether its arms are stretched out. However, the octopus has a very poor proprioceptive sense.

The tension receptors are not sufficient for the octopus brain to determine the position of the octopus's body or arms. (It is not clear that the octopus brain would be capable of processing the large amount of information that this would require; the flexibility of an octopus's arms is much greater than that of the limbs of vertebrates, which devote large areas of cerebral cortex to the processing of proprioceptive inputs.) As a result, the octopus does not possess stereognosis; that is, it does not form a mental image of the overall shape of the object it is handling. It can detect local texture variations, but cannot integrate the information into a larger picture.

The neurological autonomy of the arms means that the octopus has great difficulty learning about the detailed effects of its motions. The brain may issue a high-level command to the arms, but the nerve cords in the arms execute the details. There is no neurological path for the brain to receive feedback about just how its command was executed by the arms; the only way it knows just what motions were made is by observing the arms visually.

Locomotion

Octopuses move about by crawling or swimming. Their main means of slow travel is crawling, with some swimming. Their only means of fast travel is called jet propulsion.

They crawl by walking on their arms, usually on many at once, on solid surfaces, while supported in water. In 2005 it was reported that some octopuses can walk on two arms on a solid surface, while at the same time imitating a coconut or a clump of seaweed.

They swim by expelling a jet of water from a contractile mantle, and aiming it via a muscular siphon.

Terminology

According to various sources, there are up to three forms of the plural of octopus: octopuses, octopi and (rare) octopodes. Octopuses is the most common form, but Merriam-Webster and other dictionaries accept octopi as an alternative form, though this uses the Latinate suffix -i on a Greek morpheme -pus and is therefore deemed to be technically incorrect.

The Oxford English Dictionary lists octopuses, octopi, and octopodes in order of descending frequency of use. The term octopod (either plural octopods and octopodes can be found) is taken from the taxonomic order octopoda but has no classical equivalent. The collective form octopus is usually reserved for animals consumed for food.

Fowler's Modern English Usage states that "the only acceptable plural in English is octopuses", and that octopi is misconceived and octopodes pedantic. Octopi derives from the mistaken notion that octopus is a second declension Latin noun, which it is not. Rather, it is (Latinized) Greek, from okt?pous (??t?p???), gender masculine, whose plural is okt?podes (??t?p?de?).

If the word were native to Latin, it would be octopes ('eight-foot') and the plural octopedes, analogous to centipedes and millipedes, as the plural form of pes ('foot') is pedes. In modern, informal Greek, it is called chtapódi (?tap?d?), gender neuter, with plural form chtapódia (?tap?d?a).

Swarming of honey bees is a more specific term, referring to the reproductive action of an entire colony of bees (as opposed to the reproduction of single bees); see Queen bee

and Honey bee life cycle.

Fish

Shoal can describe any group of fish, including mixed-species groups, reserving "school" for more closely knit groups of the same species swimming in a highly synchronized and polarized manner.

Fish derive many benefits from shoaling behaviour including defense against predators (by enhanced predator detection and diluting the chance of capture), enhanced foraging success, and higher success of finding a mate. It is also likely that fish benefit from shoal membership through increased hydrodynamic efficiency One feature of a shoal is the strong resemblance between member fish. Fish use many traits to choose shoal mates including size of shoal, species type, body size, health of shoal members, and kinship.

Fish often choose to be in a shoal that consists of individuals similar in appearance to themselves; the "oddity effect" posits that any shoal member that stands out in appearance will be preferentially targeted by predators. The oddity effect would therefore tend to homogenize shoals.

One puzzling aspect of shoal selection is how a fish can choose to join a shoal of animals of similar appearance, given that it cannot know its own colour. Experiments with zebrafish have shown that shoal preference is a learned ability, not innate. A zebrafish tends to associate with shoals that resemble shoals that it was reared in (that is, a form of imprinting).

Other open questions of shoaling behaviour include determining the direction of shoal movement. In the case of Migratory movement, most members of a shoal seem to know where they are going, but foraging behaviour is more problematic. Animal behaviourist Stephan G. Reebs, writing in the journal Animal Behaviour, argues that shoals of golden shiner (a kind of minnow) were led by a small number of more experienced individuals.

In the United Kingdom, Atlantic cod is one of the most common kinds of fish to be found in fish and chips, along with haddock and plaice.

It is also well known for being largely consumed in Portugal, where it is considered a treasure of the nation's cuisine. It is an important link in the food chain.

Species in genus Gadus

At various times in the past, a very considerable number of species have been classified in this genus. However the great majority of them are now either classified in other genera, or have been recognised as simply forms of one of three species. Modern taxonomy, therefore, recognises only three species in this genus:

Atlantic cod, Gadus morhua

Pacific cod, Gadus macrocephalus

Greenland cod, Gadus ogac

All these species have a profusion of common names, most of them including the word "cod". Many common names have been used of more than one species, in different places or at different times.

Related species called cod

Cod forms part of the common name of many other fish no longer classified in the genus Gadus. Many of these are members of the family Gadidae, and several were formerly classified in genus Gadus; others are members of three related families whose names include the word "cod": the morid cods, Moridae (100 or so species); the eel cods, Muraenolepididae (4 species); and the Eucla cod, Euclichthyidae (1 species). The tadpole cod family (Ranicipitidae) has now been absorbed within Gadidae.

Species within the order Gadiformes that are commonly called cod include:

Arctic cod Arctogadus glacialis

East Siberian cod Arctogadus borisovi

Saffron cod Eleginus gracilis

Polar cod Boreogadus saida

Rock cod Lotella rhacina

Poor cod Trisopterus minutus

Pelagic cod Melanonus gracilis

Small-headed cod Lepidion microcephalus

Tadpole cod Guttigadus globosus

Eucla cod Euclichthys polynemus

Some other related fish have common names derived from "cod", such as codling, codlet or tomcod. ("Codling" is also used as a name for a young cod.)

Unrelated species called cod

However there are also fish commonly known as cod that are quite unrelated to the genus Gadus. Part of this confusion of names is market-driven. Since the decline in cod stocks has made the Atlantic cod harder to catch, cod replacements are marketed under names of the form "x cod", and culinary rather than phyletic similarity has governed the emergence of these names. A very large number of fish have thus been named as some kind of cod at some time. The following species, however, seem to have well established common names including the word "cod"; note that all are Southern Hemisphere species.

Perciformes

Fish of the order Perciformes that are commonly called "cod" include:

Murray cod Maccullochella peelii peelii

Eastern freshwater cod Maccullochella ikei

Mary River cod Maccullochella peelii mariensis

Trout cod Maccullochella macquariensis

Sleepy cod Oxyeleotris lineolatus

Blue cod Parapercis colias

The cod icefish family, Nototheniidae, including:

Black cod Paranotothenia microlepidota

Maori cod Paranotothenia magellanica

Antarctic cod Dissostichus mawsoni

Rock cod, reef cod, and coral cod

Almost all the fish known as coral cod, reef cod or rock cod are also in order Perciformes. Most are better known as groupers, and belong to the family Serranidae. Others belong to the Nototheniidiae. Two exceptions are the Australasian red rock cod, which belongs to a different order (see below), and the fish known simply as the rock cod also by soft cod in New Zealand, Lotella rhacina, which as noted above actually is related to the true cod (it is a morid cod).

Scorpaeniformes

From the order Scorpaeniformes:

Ling cod Ophiodon elongatus

Red rock cod Scorpaena papillosa

Ophidiiformes

The tadpole cod family, Ranicipitidae, and the Eucla cod family, Euclichthyidae, were formerly classified in the order Ophidiiformes, but are now grouped with the Gadiformes.

Species marketed as cod

Some fish that do not have "cod" in their names are sometimes sold as cod. Haddock and whiting belong in the same family, the Gadidae, as cod.

Haddock Melanogrammus aeglefinus

Whiting Merlangius merlangus

Identification

Classic codfish shape, with three rounded dorsal and two anal fins. The pelvic fins are small with the first ray extended, and are set under the gill cover (ie the throat region), in front of the pectorals. The upper jaw extends over the lower jaw, which has a well developed chin barbel. Medium sized eyes, approximately the same as the length of the chin barbel.

It has a distinct white lateral line running from the gill slit above the pectoral fin, to the base of the caudal or tail fin. The back tends to be a greenish to sandy brown, and showing extensive mottling especially towards the lighter sides and white belly. Dark brown colouration of the back and sides is not uncommon especially for individuals who have resided in rocky inshore regions.

Breeding

The Cod population comprises of a number of reasonably distinct stocks over its range. These include the Arcto-Norwegian, North Sea, Faroe, Iceland, East Greenland, West Greenland, Newfoundland, and Labrador stocks. There would seem to be little interchange between the stocks, although migrations to their individual breeding grounds may involve distances of 200 miles or more.

Spawning occurs between January to April (March and April are the peak months), at a depth of 200m in specific spawning grounds at water temperatures of between 4-6oC. Around the UK, the major ones are associated with the Middle to Southern North Sea, the start of the Bristol Channel (north of Newquay), the Irish Channel (both east and west of the Isle of Man), around Stornoway, and east of Helmsdale.

Pre-spawning courtship involves fin displays, and male grunting, which leads to pairing. The male is inverted underneath the female, whilst the pair swim in circles during the spawning process. The eggs are planktonic and hatch between 8 to 23 days with the larva being some 4mm in length. This planktonic phase lasts some ten weeks, during which the young cod will increase it's body weight by 40 times, and be about 2cm in length.

The young cod move to the seabed and their diet changes to small benthic crustaceans, such as isopods and small crabs. They increase in size to 8cm in the first six months, 14 to 18cm by the end of their first year, and some 25 to 35cm by the end of the second. This rate of growth tends to be less in individuals occupying northerly grounds.Cod reach maturity at about 50cm in length at about 3 to 4 years of age.

Habitat

Varied, although often favouring rough ground especially inshore. Demersal in depths of between 20 to 200m (80m Av.), although not uncommon to depths of 600m. Gregarious and forms schools, although shoaling tends to be a feature of the spawning season.

"mackerel" is the king mackerel (Scomberomorus cavalla) which can grow to 66 inches (1.68 m). Common features of mackerels are a slim, cylindrical shape (as opposed to the tunas which are deeper bodied) and numerous finlets on the dorsal and ventral sides behind the dorsal and anal fins.

The scales are extremely small, if present. They are prized (and are highly harvested) for their meat, which is often very oily, are known for their fighting ability, and are an important recreational and commercial fishery.

The meat can spoil quickly, especially in the tropics, causing scombroid food poisoning - it must be eaten on the day of capture, unless cured. For this reason, mackerel is the only fish traditionally sold on a Sunday in London, and is the only common salt-cured sushi.This fish is highly valued by fisheries.

freshwater pike, green pike or pickerel

large California rockfish

A group of marine fish in the Carangidae family

Leather jack

Crevalle jack

Yellow jack

Bar jack

Island jack

Green jack

Black jack

Fortune jack

Almaco jack

Giant trevally

A common name for the Coho salmon

A Jack salmon may refer to the small percentage of Salmon returning to fresh water from the ocean after only one year.

The leatherjacket fish:

skipjack or leather jack, Oligoplites saurus, is a jack and member of the Carangidae family. Leather jack may also refer to other members of the Carangidea family, such as the pilot fish.

Leatherjack may also refer to the smooth leatherjacket, a member of the Monacanthidae family.

"Slender, compressed shape with pointed head and large jaws for its size. Leathery skin is green above and silvery on the sides. Sharp spines on dorsal and anal fins can administer very painful puncture wounds." Size is seldom above 12 inches, although somewhat larger specimens have been reported.

Crevalle jack:

Caranx hippos, is a fast, saltwater fish that can be found in inland waters along the shoreline of the western Atlantic Ocean from Nova Scotia to Uruguay and the eastern Atlantic from Portugal to Angola. It has a large rounded head with large eyes and a dark silvery body that can show hints of blue-green to green-gold. They grow to more than three feet in length, though more commonly they are between one and two and a half feet long. The fish usually weighs between three and five pounds, but a 51 pound Crevalle Jack was taken off the coast of Florida. Crevalle jacks can be poisonous to eat due to the threat of ciguatera poisoning, but they are prized as a game fish.

Crevalle jacks spawn offshore from early March to early September. When young, they run in large schools, but become solitary as they get older. They are preyed upon by many surface feeding carnivores, including finfish, such as the striped marlin, and seabirds. Crevalle jacks feed during the day and eat a variety of fish and invertebrates. Other common names for Caranx hippos include common jack, blacktailed trevally, cabalo, green jack, horse mackerel, horse-eye jack, kingfish, and trevally.

The yellow jack:

(Carangoides bartholomaei) is one of several types of jacks. It inhabits the inland waters of the western Atlantic Ocean along the coast from Massachusetts in the United State to Brazil. These fish have a deep body that ranges in color from blue to gray to yellow. The name comes from the yellow fins that the adults typically have. Fullgrown yellow jacks can get around 30 inches long and can weigh up to 20 pounds.

families Gadidae (subfamily Phycinae)

families Merlucciidae (both subfamilies Merlucciinae and Steindachneriinae).

An old European source mentions a hake that was transplanted from the coast of Ireland to Cape Cod. It is uncertain which species this is, but the reference is given below:

This is an Irish salt water fish, similar in appearance to the tom cod. In Galway bay, and other sea inlets of Ireland, the hake is exceedingly abundant, and is taken in great numbers. It is also found in England and France. Since the Irish immigration to America, the hake has followed in the wake of their masters, as it is now found in New York bay, in the waters around Boston, and off Cape Cod.

Here it is called the stock fish, and the Bostonians call them poor Johns. It is a singular fact that until within a few years this fish was never seen in America. It does not grow so large here as in Europe, though here they are from ten to eighteen inches [250 to 460 mm] in length. The general color of this fish is a reddish brown, with some golden tints - the sides being of a pink silvery luster.

Hake is very popular in Spain, where it is known as merluza.

Species

There are five species in two genera:

Genus Anarhichas

Northern wolffish, Anarhichas denticulatus Krøyer, 1845.

Atlantic wolffish, Anarhichas lupus Linnaeus, 1758.

Spotted wolffish, Anarhichas minor Olafsen, 1772.

Bering wolffish, Anarhichas orientalis Pallas, 1814.

Genus Anarrhichthys

Wolf-eel, Anarrhichthys ocellatus Ayres, 1855.

Batoids are most closely related to sharks and young batoids

look very much like young sharks. Indeed according to recent DNA analyses the catshark is more closely related to the batoids than to other sharks.

Anatomy

Batoids are flat-bodied, and, like sharks, are a species of cartilaginous marine fish, meaning they have a boneless skeleton made of a tough, elastic substance. Batoids also are like sharks in having slot-like body openings called gill slits that lead from the gills. Batoid gill slits lie under the pectoral fins on the underside, whereas a shark's are on the sides of the head. Most batoids have a flat, disk-like body, with the exception of the guitarfishes and sawfishes, while most sharks have a streamlined body. Many species of batoid have developed their pectoral fins into broad flat wing-like appendages.

The eyes and spiracles are located on top of the head.

Reproduction

Batoid eggs, unlike those of most other fishes, are fertilized inside the female's body. The eggs of all batoids except for the skates (family Rajidae) hatch inside the female and are born alive (ovoviviparous). Female skates lay internally fertilized flat, rectangular, leathery-shelled eggs, with tendrils at the corners for anchorage. Hatched eggs of this type can be found on beaches and are known as mermaids’ purses.

Habitat

Most species live on the sea floor, in a variety of geographical regions - many in coastal waters, few live in deep waters, most batoids have a somewhat cosmopolitan distribution, in tropical and subtropical marine environments, temperate or cold-water species. Only a few species, like manta rays, live in the open sea, and only a few live in freshwater. Some batoids can live in brackish bays and estuaries. Bottom-dwelling batoids breathe by taking water in through the spiracles, rather than through the mouth as most fishes do, and passing it outward through the gills.

Feeding

Most batoids have developed heavy, rounded teeth for crushing the shells of bottom-dwelling species such as snails, clams, oysters, crustaceans, and some fish, depending on the species. Manta rays feed on plankton.

Classification

The classification of batoids is currently undergoing revision. This article follows FishBase in dividing batoids into three orders. Some taxonomists argue in favour of placing all batoids in a single order, Rajiformes; others propose a division into five or six orders. The additional orders in these systems are Myliobatiformes, containing the eagle rays and their relatives; Rhinobatiformes, containing the guitarfishes (which may be further split into Rhynchobatiformes, containing the shovelnosed guitarfishes, and Rhiniformes, the sharkfin guitarfishes).

Order Rajiformes (true rays)

Family Anacanthobatidae (smooth skates)

Family Dasyatidae (stingrays). Named for the venomous spines along the tail; these contain a poison that causes pain and may cause symptoms such as nausea, vomiting, fever, chills, muscle cramps, tremors, paralysis, fainting, seizures, elevated heart rate, and decreased blood pressure (depending on the species). In addition, some species' toxins can be fatal to humans.

Family Gymnuridae (butterfly rays)

Family Hexatrygonidae (sixgill stingrays)

Family Myliobatidae (eagle rays). The largest of rays, including the giant manta rays. Most eagle rays have one poison-carrying spine.

Family Plesiobatidae (deepwater stingrays)

Family Potamotrygonidae (river stingrays)

Family Rajidae (skates)

Family Rhinobatidae (guitarfishes). They have a body structure similar that of the sawfishes, but are not thought to be closely related.

Family Urolophidae (round rays)

Order Pristiformes (sawfishes)

Sawfishes are shark-like in form, having tails used for swimming and smaller pectoral fins than most batoids. The pectoral fins are attached above the gills as in all batoids, giving the fishes a broad-headed appearance. They have long, flat snouts with a row of tooth-like projections on either side. The snouts are up to 6 ft (1.8 m) long, and 1 ft (30 cm) wide, and are used for slashing and impaling small fishes and to probe in the mud for imbedded animals. Sawfishes can enter freshwater rivers and lakes. Some species reach a total length of 20 ft (6 m).

Family Pristidae

Order Torpediniformes (electric rays)

Electric rays have organs in their wings that generate electric current. They are used to immobilize prey and for defense. The current is strong enough to stun humans, and it is said that the ancient Greeks used these fish for shock therapy.

Family Narcinidae

Family Torpedinidae

The Humpback Whale, Megaptera novaeangliae, is a baleen whale. One of the larger rorqual species, adults range in length from 12–16 metres (40–50 ft) and weigh approximately 36,000 kilograms (79,000 lb).

The Humpback has a distinctive body shape, with unusually long pectoral fins and a knobbly head. It is an acrobatic animal, often breaching and slapping the water. Males produce a complex whale song, which lasts for 10 to 20 minutes and is repeated for hours at a time. The purpose of the song is not yet clear, although it appears to have a role in mating.

Found in oceans and seas around the world, Humpback

Whales typically migrate up to 25,000 kilometres each year. Humpbacks feed only in summer, in polar waters, and migrate to tropical or sub-tropical waters to breed and give birth in the winter. During the winter, Humpbacks fast and live off their fat reserves. The species' diet consists mostly of krill and small fish. Humpbacks have a diverse repertoire of feeding methods, including the spectacular bubble net fishing technique.

Like other large whales, the Humpback was a target for the whaling industry, and its population fell by an estimated 90% before a whaling moratorium was introduced in 1966.

Stocks of the species have since partially recovered, however entanglement in fishing gear, collisions with ships, and noise pollution are ongoing concerns. Current estimates for the abundance of Humpback Whales range from about 30,000 to 60,000, approximately one third of pre-whaling levels. Once hunted to the brink of extinction, Humpbacks are now sought out by whale-watchers, particularly off parts of Australia and the United States.

Taxonomy

Humpback Whales are rorquals (family Balaenopteridae), a family that includes the Blue Whale, the Fin Whale, the Bryde's Whale, the Sei Whale and the Minke Whale. The rorquals are believed to have diverged from the other families of the suborder Mysticeti as long ago as the middle Miocene. However, it is not known when the members of these families diverged from each other.

Though clearly related to the giant whales of the genus Balaenoptera, the Humpback has been the sole member of its genus since Gray's work in 1846. More recently though, DNA sequencing analysis has indicated both the Humpback and the Gray Whale are close relatives of the Blue Whale, the world's largest animal. If further research confirms these relationships, it will be necessary to reclassify the rorquals.

The Humpback Whale was first identified as "baleine de la Nouvelle Angleteer" by Mathurin Jacques Brisson in his Regnum Animale of 1756. In 1781, Georg Borowski described the species, converting Brisson's name to its Latin equivalent, Baleana novaeangliae. Early in the 19th century Lacépède shifted the Humpback from the Balaenidae family, renaming it Balaenoptera jubartes.

In 1846, John Edward Gray created the genus Megaptera, classifying the Humpback as Megaptera longpinna, but in 1932, Remington Kellogg reverted the species names to use Borowski's novaeangliae. The common name is derived from their humping motion while swimming. The generic name Megaptera from the Greek mega-/µe?a- "giant" and ptera/pte?a "wing", refers to their large front flippers. The specific name means "New Englander" and was probably given by Brisson due the regular sightings of Humpbacks off the coast of New England.

Description and lifecycle

Humpback Whales can easily be identified by their stocky bodies with obvious humps and black dorsal colouring. The head and lower jaw are covered with knobs called tubercles, which are actually hair follicles and are characteristic of the species. The tail flukes, which are lifted high in the dive sequence, have wavy rear edges.

The long black and white tail fin, which can be up to a third of body length, and the pectoral fins have unique patterns, which enable individual whales to be recognised. Several suggestions have been made to explain the evolution of the Humpback's pectoral fins, which are proportionally the longest fins of any cetacean.

The two most enduring hypotheses are the higher maneuverability afforded by long fins, or that the increased surface area is useful for temperature control when migrating between warm and cold climates. Humpbacks have 270 to 400 darkly coloured baleen plates on each side of the mouth. Ventral grooves run from the lower jaw to the umbilicus about halfway along the bottom of the whale.

These grooves are less numerous (usually 16–20) and consequently more prominent than in other rorquals. The stubby dorsal fin is visible soon after the blow when the whale surfaces, but has disappeared by the time the flukes emerge. Humpbacks have a distinctive 3 m (10 ft) bushy blow.

Calves are about 4–4.5 m (13–15 ft) long when born and weigh approximately 700 kg (1500 lb). They are nursed by their mothers for their first six months, then are sustained through a mixture of nursing and independent feeding for a further six months. Calves leave their mothers at the start of their second year, when they are typically 9 m (30 ft) long.

Both sexes reach sexual maturity at the age of five with full adult size being achieved a little later. Fully grown the males average 15–16 m (49–52 ft), the females being slightly larger at 16–17 m (52–56 ft), with a weight of 40,000 kg (or 44 tons); the largest recorded specimen was 19 m (62 ft) long and had pectoral fins measuring 6 m (20 ft) each.

Females have a lobe about 15 centimetres (6 in) in diameter in their genital region. This allows males and females to be distinguished if the underside of the whale can be seen, even though the male's penis usually remains unseen in the genital slit. Male whales have distinctive scarring patterns and pigmentations on their underside, some resulting from high speed courtship chases of females.

Females typically breed every two or three years. The gestation period is 11 months, yet some individuals can breed in two consecutive years. Humpback Whales can live for 40–50 years.

Identification

The varying patterns on the Humpback's tail flukes are sufficient to identify an individual. Unique visual identification is not possible in most cetacean species (exceptions include Orcas and Right Whales), so the Humpback has become one of the most-studied species. A study using data from 1973 to 1998 on whales in the North Atlantic gave researchers detailed information on gestation times, growth rates, and calving periods, as well as allowing more accurate population predictions by simulating the mark-release-recapture technique.

A photographic catalogue of all known whales in the North Atlantic was developed over this period and is currently maintained by Wheelock College.[9] Similar photographic identification projects have subsequently begun in the North Pacific by SPLASH (Structure of Populations, Levels of Abundance and Status of Humpbacks), and around the world.

Social structure and courtship

The Humpback social structure is loose-knit. Usually, individuals live alone or in small transient groups that assemble and break up over the course of a few hours. Groups may stay together a little longer in summer in order to forage and feed cooperatively. Longer-term relationships between pairs or small groups, lasting months or even years, have been observed, but are rare. The range of the Humpback overlaps considerably with many other whale and dolphin species — whilst it may be seen near other species (for instance, the Minke Whale), it rarely interacts socially with them.

Courtship rituals take place during the winter months, when the whales migrate towards the equator from their summer feeding grounds closer to the poles. Humpback Whales do not feed while in their wintering waters. Competition for a mate is usually fierce, and female whales as well as mother-calf dyads are frequently trailed by unrelated male whales dubbed escorts by researcher Louis Herman.

Groups of two to twenty males typically gather around a single female and exhibit a variety of behaviours in order to establish dominance in what is known as a competitive pod. The displays last several hours, the group size may ebb and flow as unsuccessful males retreat and others arrive to try their luck. Techniques used include breaching, spy-hopping, lob-tailing, tail-slapping, flipper-slapping, charging and parrying.

Whale song is assumed to have an important role in mate selection; however, scientists remain unsure whether the song is used between males in order to establish identity and dominance, between a male and a female as a mating call, or a mixture of the two. All these vocal and physical techniques have also been observed while not in the presence of potential mates. This indicates that they are probably important as a more general communication tool.

Feeding

The species feeds only in summer and lives off fat reserves during winter. It is an energetic feeder, taking krill and small schooling fish, such as herring (Clupea harengus), salmon, capelin (Mallotus villosus) and sand lance (Ammodytes americanus) as well as Mackerel (Scomber scombrus), pollock (Pollachius virens) and haddock (Melanogrammus aeglefinus) in the North Atlantic. Krill and Copepods have been recorded from Australian and Antarctic waters. It hunts fish by direct attack or by stunning them by hitting the water with its flippers or flukes.

The Humpback has the most diverse repertoire of feeding methods of all baleen whales.[14] Its most inventive technique is known as bubble net fishing: a group of whales blows bubbles while swimming to create a visual barrier against fish, while one or more whales in the group make vocalizations that drive the fish against the wall.

The bubble wall is then closed, encircling the fish, which are confined in an ever-tighter area. The whales then suddenly swim upwards through the bubble net, mouths agape, swallowing thousands of fish in one gulp. This technique can involve a ring of bubbles up to 30 m (100 ft) in diameter and the cooperation of a dozen animals. It is one of the more spectacular acts of collaboration among marine mammals.

Humpback Whales are preyed upon by Orcas. The result of these attacks is generally nothing more serious than some scarring of the skin, but it is likely that young calves are sometimes killed.

Song

Both male and female Humpback Whales can produce sounds, however only the males produce the long, loud, complex "songs" for which the species is famous. Each song consists of several sounds in a low register that vary in amplitude and frequency, and typically lasts from 10 to 20 minutes.

Songs may be repeated continuously for several hours; Humpback Whales have been observed to sing continuously for more than 24 hours at a time. As cetaceans have no vocal chords, whales generate their song by forcing air through their massive nasal cavities.

Whales within an area sing the same song, for example all of the Humpback Whales of the North Atlantic sing the same song, and those of the North Pacific sing a different song. Each population's song changes slowly over a period of years —never returning to the same sequence of notes.

Scientists are still unsure of the purpose of whale song. Only male Humpbacks sing, so it was initially assumed that the purpose of the songs was to attract females. However, many of the whales observed to approach singing whales have been other males, with the meeting resulting in a conflict.

Thus, one interpretation is that the whale songs serve as a threat to other males. Some scientists have hypothesized that the song may serve an echolocative function. During the feeding season, Humpback Whales make altogether different vocalizations, which they use to herd fish into their bubble nets.

Population and distribution

The Humpback Whale is found in all the major oceans, in a wide band running from the Antarctic ice edge to 65° N latitude, though is not found in the eastern Mediterranean, the Baltic Sea or the Arctic Ocean. Estimating cetacean poplation levels is difficult; current estimates for Humpbacks are in the 30,000[16] to 60,000 range, down from a pre-whaling population of 125,000.

The Humpback is a migratory species, spending its summers in cooler, high-latitude waters, but mating and calving in tropical and sub-tropical waters. An exception to this rule is a population in the Arabian Sea, which remains in these tropical waters year-round. Annual migrations of up to 25,000 kilometres (16,000 statute miles) are typical, making it one of the farthest-travelling of any mammalian species.

A 2007 study identified seven individual whales wintering off the Pacific coast of Costa Rica as those which had made a trip from the Antarctic of around 8300 km. Identified by their unique tail patterns, these animals have made the longest documented migration by a mammal.

In Australia, two main migratory populations have been identified, off the west and east coast respectively. These two populations are distinct with only a few females in each generation crossing between the two groups.

Whaling

The first recorded Humpback kill was made in 1608 off Nantucket. Opportunistic killing of the species is likely to have occurred long before, and it continued with increasing pace in the following centuries. By the 18th century, the commercial value of Humpback Whales had been recognized, and they became a common target for whalers for many years.

By the 19th century, many nations (and the United States in particular), were hunting the animal heavily in the Atlantic Ocean — and to a lesser extent in the Indian and Pacific Oceans. However, it was the introduction of the explosive harpoon in the late 19th century that allowed whalers to accelerate their take. This, coupled with the opening-up of the Antarctic seas in 1904, led to a sharp decline in all whale populations.

It is estimated that during the 20th century at least 200,000 Humpbacks were taken, reducing the global population by over 90%, with the population in the North Atlantic estimated to have dropped to as low as 700 individuals.

To prevent species extinction, a general moratorium on the hunting of Humpbacks was introduced in 1966 and is still in force today. In his book Humpback Whales (1996), Phil Clapham, a scientist at the Smithsonian Institute, said "This wanton destruction of some of the earth's most magnificent creatures one of the greatest of our many environmental crimes."

By the time the International Whaling Commission (IWC) members agreed on a moratorium on Humpback hunting in 1966, the whales were so scarce that commercial hunting was no longer worthwhile. At this time, 250,000 were recorded killed. However, the true toll is likely to be significantly higher. It is now known that the Soviet Union was deliberately under-recording its kills; the total Soviet Humpback kill was reported at 2,820 whereas the true number is now believed to be over 48,000.

As of 2004, hunting of Humpback Whales is restricted to a few animals each year off the Caribbean island Bequia in the nation of St. Vincent and the Grenadines. The take is not believed to threaten the local population.

2007 Japanese whaling

Starting in November 2007, Japan is planning to kill 50 Humpback Whales a year in the Southern Ocean Whale Sanctuary under its JARPA-II research program. The announcement sparked global protests.

In New Zealand, protests have come from Maori and Pacific community leaders. Whales hold a significant place in the tradition and culture of many Pacific countries, according to Melino Maka, chairman of the Tongan Advisory Council. "We have a spiritual connection with our whales in our waters." he said.

Protests occurred 20 centres around Australia as well as Tonga. Many whales known to locals and tourism operators in Australian waters were born after whaling finished, so around humans they're benign. Japan's resumption of whaling may cause the remaining animals to become nervous, agitated or belligerent around humans and vessels. It is feared this will damage tourism.

The Australian government has been vocal in its opposition to whaling, but has been criticized for not taking legal action against it. The Australian shadow environment minister, Peter Garrett, has announced a policy whereby Australian navy ships would intercept and board whaling vessels in the lead up to the Federal election. Whale watching is worth an estimated $260 million in Australia.

Anti-whaling commercials with the slogan "Tell Japan We'll Keep the Ban", narrated by Sir Trevor McDonald, were launched in the Caribbean by Lord Ashcroft, the deputy chairman of the Conservative Party. The Antiguan and Dominican governments have blocked the ad from being shown on their state owned channels, as has the MTV's Tempo network across the Carabian. The ad is being broadcast in Antigua & Barbuda, Dominica, Grenada, Saint Kitts & Nevis, Saint Lucia and Saint Vincent & the Grenadines.

There are only around 2,000 humpbacks in the entire South Pacific. The local populations are critically endangered in Fiji and Samoa. Whaling may also cause naturally isolated populations to mix, reducing distinct genetic groups.

Conservation

Internationally this species is considered vulnerable. Most monitored stocks of Humpback Whales have rebounded well since the end of the commercial whaling era. However, the species is considered endangered in some countries where local populations have recovered slowly, including the United States.

Today, individuals are vulnerable to collisions with ships, entanglement in fishing gear, and noise pollution. Like other cetaceans, Humpbacks are sensitive to noise and can even be injured by it. In the 19th century, two Humpback Whales were found dead near sites of repeated oceanic sub-bottom blasting, with traumatic injuries and fractures in the ears.

The ingestion of saxitoxin, a PSP (paralytic shellfish poison) from contaminated mackerel has been implicated in Humpback Whale deaths.

Some countries are creating action plans to protect the Humpback; for example, in the United Kingdom, the Humpback Whale has been designated as a priority species under the national Biodiversity Action Plan, generating a set of actions to conserve the species. The sanctuary provided by National Parks such as Glacier Bay National Park and Preserve and Cape Hatteras National Seashore, among others, have also become a major factor in sustaining the populations of the species in those areas.

Although much was known about the Humpback Whale due to information obtained through whaling, the migratory patterns and social interactions of the species were not well known until two separate studies by R. Chittleborough and W. H. Dawbin in the 1960s. Roger Payne and Scott McVay made further studies of the species in 1971. Their analysis of whale song led to worldwide media interest in the species, and left an impression in the public mind that whales were a highly intelligent cetacean species, a contributing factor to the anti-whaling stance of many countries.

Whale-watching

Humpback Whales are generally curious about objects in their environment. They will often approach and circle boats. Whilst this inquisitiveness was akin to suicide when the vessel was a whaling ship, it has become an attraction of whale-watching tourism in many locations around the world since the 1990s.

Whale-watching locations include the Atlantic coast off the Samana province of the Dominican Republic, the Pacific coast off Oregon, Washington, Vancouver, Hawaii and Alaska, the Bay of Biscay to the west of France, Byron Bay north of Sydney, Hervey Bay north of Brisbane, the coasts of New England and Newfoundland, New Zealand, the Tongan islands, the northern St. Lawrence River and the Snaefellsnes peninsula in the west of Iceland. The species is popular because it breaches regularly and spectacularly, and displays a range of other social behaviours.

As with other cetacean species, however, a mother whale will generally be extremely protective of her infant, and will seek to place herself between any boat and the calf before moving quickly away from the vessel. Whale-watching tour operators are asked to avoid stressing the mother.

Blue Whales were abundant in nearly all oceans around the world until the beginning of the twentieth century.

For the first 40 years of the century they were hunted by whalers almost to extinction. Hunting of the species was outlawed by the international community in 1966. A 2002 report estimated there were 5,000 to 12,000 Blue Whales worldwide located in at least five groups. More recent

research into the Pygmy subspecies suggest this may be an underestimate. Before whaling the largest population 239,000 (range 202,000 to 311,000) was in the Antarctic but now there remain only much smaller (around 2,000) concentrations in each of the North-East Pacific, the Antarctic, and the Indian Ocean. There are two more groups in the North Atlantic and at least two in the Southern Hemisphere.

Taxonomy and evolution

Blue Whales are rorquals (family Balaenopteridae), a family that includes the Humpback Whale, the Fin Whale, the Bryde's Whale, the Sei Whale and the Minke Whale. The family Balaenopteridae is believed to have diverged from the other families of the suborder Mysticeti as long ago as the middle Oligocene. However, it is not known when the members of those families diverged from each other.

The Blue Whale is usually classified as one of seven species of whale in the genus Balaenoptera; however, DNA sequencing analysis indicates that Blue Whales are phylogenetically closer to the Humpback (Megaptera) and the Gray Whale (Eschrichtius) than to other Balaenoptera species; should further research corroborate these relationships, it will be necessary to recognize the separate genus Sibbaldus for the Blue Whale.

There have been at least 11 documented cases of Blue/Fin Whale hybrid adults in the wild. Aranson and Gullberg (1983)[6] describe the genetic distance between a Blue and a Fin as about the same as that between a human and gorilla. Blue Whale/Humpback Whale hybrids are also known.

The specific name musculus is Latin and could mean "muscular", but it can also be interpreted as "little mouse". Linnaeus, who named the species in his seminal work of 1758, would have known this and, given his sense of humour, may have intended the ironic double meaning. Other common names for the Blue Whale have included the Sulphur-bottom, Sibbald's Rorqual, the Great Blue Whale and the Great Northern Rorqual. These names have fallen into disuse in recent decades.

Authorities classify the species into three subspecies: B. m. musculus, consisting of the north Atlantic and north Pacific populations, B. m. intermedia, the Southern Ocean population and B. m. brevicauda (also known as the Pygmy Blue Whale) found in the Indian Ocean and South Pacific.

Some older authorities also list B. m. indica as a further separate subspecies in the Indian Ocean, but it is most likely that these blue whales are pygmy blue whales, and this designation does not therefore have a listing in the Red List of Threatened Species. While both subdivisions are still questioned by some scientists; other scientists have suggested that South-East Pacific blue whales may also be separate subspecies.

Description and behaviour

The Blue Whale has a long tapering body that appears stretched in comparison with the much stockier appearance of other whales. The head is flat and U-shaped and has a very prominent ridge running from the blowhole to the top of the upper lips. The front part of the mouth is thick with baleen plates; around 300 plates (each one metre long) hang from the upper jaw, running half a metre back into the mouth. Between 60 and 90 grooves (called ventral pleats) run along the throat parallel to the body.

These plates assist with evacuating water from the mouth after lunge feeding (see feeding below). The dorsal fin is small, visible only briefly during the dive sequence. It varies in shape from one individual to another; some only have a barely perceptible lump, but other whales' dorsal fins are quite prominent and falcate. It is located around three-quarters of the way along the length of the body.

When surfacing to breathe, the Blue Whale raises its shoulder and blow hole region out of the water to a greater extent than other large whales (such as the Fin or Sei). This can often be a useful clue to identifying a species at sea. When breathing, the whale emits a spectacular vertical single column blow (up to 12 m, typically 9 m) that can be seen from many kilometers on a calm day. Its lung capacity is 5,000 litres.

The flippers are three to four metres long. The upper side is grey with a thin white border. The lower side is white. The head and tail fluke are generally uniformly grey coloured. The back, and sometimes the flippers, are usually mottled. The degree of mottling varies substantially from individual to individual. Some may have a uniform grey colour all over, but others demonstrate a considerable variation of dark blues, greys and blacks all tightly mottled.

Blue Whales can reach speeds of 50 km/h (30 mph) over short bursts, usually when interacting with other whales, but 20 km/h (12 mph) is a more typical travelling speed. When feeding they slow down to 5 km/h (3 mph). Some Blues in the North Atlantic and North Pacific raise their tail fluke when diving. The majority, however, do not.

Blue Whales most commonly live alone or with one other individual. It is not known whether those that travel in pairs stay together over many years or form more loose relationships. In areas of very high food concentration, as many as 50 Blue Whales have been seen scattered over a small area. However, they do not form large close-knit groups as seen in other baleen species.

Size

The Blue Whale is believed to be the largest animal ever to have lived on Earth. The largest known dinosaur of the Mesozoic era was the Argentinosaurus, which is estimated to have weighed up to 90 tonnes (100 short tons). There is some uncertainty about the biggest Blue Whale ever found. Most data comes from Blue Whales killed in Antarctic waters during the first half of the twentieth century and was collected by whalers not well-versed in standard zoological measurement techniques.

The longest whales ever recorded were two females measuring 33.6 m and 33.3 m (110 ft 3 in and 109 ft 3 in) respectively. However, there are some disputes over the reliability of these measurements. The longest whale measured by scientists at the American National Marine Mammal Laboratory (NMML) was 29.9 m long (98 ft) — about the same length as a Boeing 737 aeroplane or three double-decker buses.

A Blue Whale's tongue is about the size of an elephant and 50 humans could stand in its mouth. Although the mouth is as large as a small garage, the dimensions of its throat are such that a Blue Whale cannot swallow an object wider than a beach ball. Its heart is close to the size of a small car and is the largest known in any animal.

A human baby could squeeze into a Blue Whale's aorta, which is about 23 centimetres (9 inches) in diameter. During the first 7 months of its life, a Blue Whale calf drinks approximately 400 litres (100 US gallons) of milk every day. Blue Whale calves gain weight as quickly as 90 kg (200 pounds) every 24 hours. Even at birth, they weigh up to 2700 kilograms (6000 lb) – the same as a fully-grown hippopotamus.

Blue Whales are very difficult to weigh because of their massive size. Most Blue Whales killed by whalers were not weighed as a whole, but cut up into manageable pieces before being weighed. This caused an underestimate of the total weight of the whale, due to loss of blood and other fluids.

Nevertheless, measurements between 150 and 170 tonnes (160 and 190 short tons) were recorded of animals up to 27 m (88 ft 6 inches) in length. The weight of a 30 m (98 ft) individual is believed by the NMML to be in excess of 180 tonnes (200 short tons). The largest Blue Whale accurately weighed by NMML scientists to date was a female that weighed 177 tonnes (196 short tons).

Feeding

Blue Whales feed exclusively on krill. The exact species of this zooplankton eaten by Blue Whales varies from ocean to ocean. In the North Atlantic Meganyctiphanes norvegica, Thysanoessa raschii, Thysanoessa inermis and Thysanoessa longicaudata are the usual food. In the North Pacific Euphausia pacifica, Thysanoessa inermis, Thysanoessa longipes, Thysanoessa spinifera and Nyctiphanes symplex; in the Antarctic Euphausia superba, Euphausia crystallorophias and Euphausia valentin.

The whales always feed on the highest concentration of krill that they can find. This means that they typically feed at depth (more than 100 m) during the daytimes, and only surface feed at night. Dive times are typically ten minutes when feeding. Diving for twenty minutes is quite common. The longest recorded is thirty-six minutes (Sears 1998). The whale feeds by lunging forward at groups of krill, taking the animals and a large quantity of water into the mouth at once.

The water is then squeezed out through the baleen plates by pressing the ventral pouch and tongue up against the water. Once the mouth is clear of water, the remaining krill, unable to pass through the plates, are swallowed. According to Ted Dewan's Inside the Whale and Other Animals, as well as krill, the blue whale filters small fish and squid. It may even swallow something else that was also feeding on the krill.

Life cycle

Mating starts in late autumn, and continues to the end of winter. Little is known about mating behaviour or even breeding grounds. Females typically give birth at the start of the winter once every two to three years after a gestation period of ten to twelve months. The calf weighs about two and a half tonnes and is around 7 m in length. Blue Whale calves drink 100–150 gallons (380–570 litres) of milk a day.

Weaning takes place for about six months, by which time the calf has doubled in length. Sexual maturity is typically reached at eight to ten years by which time males are at least 20 m long (or more in the southern hemisphere). Females are larger still, reaching sexual maturity at around the age of five, by which time females are about 21 m long.

Scientists estimate that Blue Whales can live for at least eighty years; however, since individual records do not date back into the whaling era, this will not be known with certainty for many years yet.

The longest recorded study of a single individual is thirty-four years, in the north-east Pacific (reported in Sears, 1998). The whales' only natural predator is the Orca. Studies report that as many as 25% of mature Blue Whales have scars resulting from Orca attack. The rate of mortality due to such attacks is unknown.

Blue Whale strandings are extremely uncommon and, because of the species' social structure, mass strandings are unheard of. However when strandings do occur they can become quite a public event. In 1920, a Blue Whale washed up near Bragar on the Isle of Lewis in the Outer Hebrides of Scotland.

It had been shot in the head by whalers, but the harpoon had failed to explode. As with other mammals, the fundamental instinct of the whale was to try to carry on breathing at all costs, even though this meant beaching to prevent itself from drowning. Two of the whale's bones were erected just off a main road on Lewis, and remain a tourist attraction.

True oysters

The "true oyster" are the members of the family Ostreidae, and this includes the edible oysters, which mainly belong to

the Arian genera Ostrea, Crassostrea, Ostreola or Saccostrea. Examples are the Edible Oyster, Ostrea edulis, Eastern Oyster Crassostrea virginica, Olympia Oyster Ostreola conchaphila, Pacific Oyster Crassostrea gigas, Sydney rock oyster Saccostrea glomerata, and the Wellfleet oyster (a variety of C. virginica).

Physical characteristics

Oysters are filter-feeders that draw water in over their gills through the beating of cilia. Suspended food plankton and particles are trapped in the mucus of the gills and transported to the mouth, where they are eaten, digested and expelled as feces or pseudofeces. Feeding activity is greatest in oysters when water temperatures are above 50°F (10°C). Healthy oysters consume algae and other water-borne nutrients, each one filtering up to five litres of water per hour. Scientists believe that the Chesapeake Bay's once-flourishing oyster populations historically filtered the estuary's entire water volume of excess nutrients every three or four days. Today that process would take almost a year, and sediment, nutrients, and algae can cause problems in local waters. Oysters filter these pollutants, and either eat them or shape them into small packets that are deposited on the bottom where they are harmless.

Oysters breathe much like fish, using both gills and mantle. The mantle is lined with many small, thin-walled blood vessels which extract oxygen from the water and expel carbon dioxide. A small, three-chambered heart, lying under the abductor muscle, pumps colorless blood, with its supply of oxygen, to all parts of the body. At the same time two kidneys located on the underside of the muscle purify the blood of any waste products they have collected.

There is no way of determining male oysters from females by examining their shells. While oysters have separate sexes, they may change sex one or more times during their life span. The gonads, organs responsible for producing both eggs and sperm, surround the digestive organs and are made up of sex cells, branching tubules and connective tissue.

Oyster habitat and lifestyle

As a keystone species, oysters provide habitat for an extensive array of marine life. The native oyster usually inhabits water depths of between 8 and 25 feet. The hard surfaces of oyster shells and the nooks between the shells provide places where a host of small animals can live. Hundreds of animals such as anemones, barnacles, and hooked mussels use oyster reefs as habitat. Many of these animals serve as food for larger animals, including striped bass, black drum and croakers. An oyster reef can encompass 50 times the surface area of an equally extensive flat bottom. The oyster contributes to improved water quality through its filter feeding capacity. An oyster's mature shape often depends on the type of bottom to which it is originally attached. It orients itself with its outer, flared shell tilted upward. One valve is cupped and the other is flat. The submerged shell opens periodically to permit the oyster to feed.

Oysters usually mature by one year of age. They are protandric, which means that during their first year they spawn as males (releasing sperm into the water). As they grow larger over the next two or three years and develop greater energy reserves, they release eggs, as females. Bay oysters are usually prepared to spawn by the end of June. An increase in water temperature prompts a few initial oysters to spawn. This triggers a spawning 'chain reaction', which clouds the water with millions of eggs and sperm. A single female oyster can produce up to 100 million eggs annually. The eggs become fertilized in the water and develop into larvae, which eventually find suitable sites on which to settle, such as another oyster's shell. Attached oyster larvae are called 'spat'. Spat are oysters 25 mm or less in length.

Some oysters in the tropics grow on mangrove roots and are exposed at low tide making them easy to collect. In Trinidad in the West Indies tourists are often astounded when they are told that "oysters grow on trees."

Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short "tail" (Greek: brachy = short, ura = tail), or where the abdomen is entirely hidden under the thorax.

They are generally covered with a thick exoskeleton, and are armed with a single pair of chelae (claws). Crabs are found in all of the world's oceans; there are also many freshwater and terrestrial crabs, particularly in tropical regions.

Crabs vary in size from the pea crab, only a few millimetres

wide, to the Japanese spider crab, with a leg span of up to 4 m

Anatomy

True crabs have five pairs of legs, the first of which are modified into a pair of claws and are not used for locomotion. In all but a few crabs (for example, Raninoida), the abdomen is folded under the cephalothorax.

The mouthparts of crabs are covered by flattened maxillipeds, and the front of the carapace does not form a long rostrum. The gills of crabs are formed of flattened plates ("phyllobranchiate"), resembling those of shrimp, but of a different structure.

Most crabs show clear sexual dimorphism and so can be easily sexed. The abdomen, which is held recurved under the thorax, is narrow in males. In females, however, the abdomen retains a greater number of pleopods and is considerably wider .

This relates to the carrying of the fertilised eggs by the female crabs (as seen in all pleocyemates). In those species in which no such dimorphism is found, the position of the gonopores must be used instead. In females, these are on the third pereiopod, or nearby on the sternum in higher crabs; in males, the gonopores are at the base of the fifth pereiopods or, in higher crabs, on the sternum nearby.

Diet

Crabs are omnivores, feeding primarily on algae , and taking any other food, including molluscs, worms, other crustaceans, fungi, bacteria and detritus, depending on their availability and the crab species. For many crabs, a mixed diet of plant and animal matter results in the fastest growth and greatest fitness.

Crab fishery

Crabs make up 20% of all marine crustaceans caught and farmed worldwide, with over 1½ million tonnes being consumed annually. Of that total, one species accounts for one fifth: Portunus trituberculatus.

Other important taxa include Portunus pelagicus, several species in the genus Chionoecetes, the blue crab (Callinectes sapidus), Charybdis spp., Cancer pagurus, the Dungeness crab (Cancer magister) and Scylla serrata, each of which provides more than 20,000 tonnes annually.

Evolution and classification

The infraorder Brachyura contains about 70 families, as many as the remainder of the Decapoda. The evolution of crabs is characterised by an increasing robustness of the body, and a reduction in the abdomen.

Although other groups have also undergone similar processes of carcinisation, it is most advanced in crabs. The telson is no longer functional in crabs, and the uropods are absent, having probably evolved into small devices for holding the reduced abdomen tight against the sternum.

In most decapods, the gonopores (sexual openings) are found on the legs. However, since crabs use the first two pairs of pleopods (abdominal appendages) for sperm transfer, this arrangement has changed. As the male abdomen evolved into a narrower shape, the gonopores have moved towards the midline, away from the legs, and onto the sternum.

A similar change occurred, independently, with the female gonopores. The movement of the female gonopore to the sternum defines the clade Eubrachyura, and the later change in the position of the male gonopore defines the Thoracotremata. It is still a subject of debate whether those crabs where the female, but not male, gonopores are situated on the sternum form a monophyletic group.

The earliest unambiguous crab fossils date from the Jurassic, although the Carboniferous Imocaris, known only from its carapace is thought to be a primitive crab. The radiation of crabs in the Cretaceous and afterwards may be linked either to the break-up of Gondwana or to the concurrent radiation of bony fish, the main predators of crabs.

About 850 species of crab are freshwater or (semi-)terrestrial species; they are found throughout the world's tropical and semi-tropical regions. They were previously thought to be a closely related group, but are now believed to represent at least two distinct lineages, one in the Old World and one in the New World.

Distribution and habitat

Great white sharks live in almost all coastal and offshore waters which have a water temperature of between 12 and 30° C (54° to 75° F), with greater concentrations off the southern coasts of Australia, off South Africa, California, Mexico's Isla Guadalupe and to a degree in the Central Mediterranean and Adriatic Seas. One of the densest known populations is found around Dyer Island, South Africa where much research on the shark is conducted.

It can be also found in tropical waters like those of the Caribbean and has been recorded off Mauritius. It is a pelagic fish, but recorded or observed mostly in coastal waters in the presence of rich game like fur seals, sealions, cetaceans, other sharks and large bony fish species. It is considered an open-ocean dweller and is recorded from the surface down to depths of 1,280 metres (4,200 ft), but is most often found close to the surface.

In a recent study great white sharks from California were shown to migrate to an area between Baja California and Hawaii, where they spend at least 100 days of the year before they migrate back to Baja. On the journey out, they swim slowly and dive to up to 900 metres (3,000 ft). After they arrive, they change behaviour and do short dives to about 300 m (1,000 ft) for up to 10 minutes. It is still unknown why they migrate and what they do there; it might be seasonal feeding or possibly a mating area.

In a similar study a great white shark from South Africa was tracked swimming to the northwestern coast of Australia and back to the same location in South Africa, a journey of 20,000 kilometres in under 9 months.

Anatomy and appearance

The great white shark has a robust large conical-shaped snout. It has almost the same size upper and lower lobes on the tail fin (like most mackerel sharks, but unlike most other sharks). It is pale to dark grey and has a white stomach.

Great white sharks have a white belly and a grey back. The colouration makes it difficult for prey to spot the shark because it breaks up the shark's outline when seen from a lateral perspective. When viewed from above, the darker shade blends in with the sea.

Great white sharks, like many other sharks, have rows of teeth behind the main ones, allowing any that break off to be rapidly replaced. Their teeth are unattached to the jaw and are retractable, like a cat's claws, moving into place when the jaw is opened. Their teeth also rotate on their own axis (outward when the jaw is opened, inward when closed). The teeth are linked to pressure and tension-sensing nerve cells.

This arrangement seems to give their teeth high tactile sensitivity. A great white shark's teeth are serrated and when the shark bites it will shake its head side to side and the teeth will act as a saw and tear off large chunks of flesh. Great white sharks often swallow their own broken off teeth along with chunks of their prey's flesh. These teeth frequently cause damage to the great white shark's digestive tract. However great white sharks often feed on stingrays and swallow the 'sting' as well, the barbed sting often getting stuck in the shark's intestines.

Size

The average length of a full-grown great white shark is 4 to 4.8 metres (13.3 to 15.8 ft), with a weight of 680 to 1,100 kilograms (1,500 to 2,450 lbs), females generally being larger than males. But the question of the maximum size of a great white shark has been subject to much debate, conjecture, and misinformation. Richard Ellis and John E. McCosker, both academic shark experts, devote a full chapter in their book, The Great White Shark (1991), to analysing various accounts of extreme size.

Today, most experts contend that the great white shark's "normal" maximum size is about 6 metres (20 ft), with a maximum weight of about 1,900 kilograms (4,200 lb).

For some decades many ichthyological works, as well as the Guinness Book of World Records, listed two great white sharks as the largest individuals caught: an 11 metre (36 ft) great white captured in south Australian waters near Port Fairy in the 1870s, and an 11.3 metre (37.6 ft) shark trapped in a herring weir in New Brunswick, Canada in the 1930s. While this was the commonly accepted maximum size, reports of 7.5 to 10 metre (25 to 33.3 ft) great white sharks were common and often deemed credible.

Some researchers questioned the reliability of both measurements, noting they were much larger than any other accurately-reported great white shark. The New Brunswick shark may have been a wrongly-identified basking shark, as both sharks have similar body shapes. The question of the Port Fairy shark was settled in the 1970s, when J.E. Reynolds examined the shark's jaws and "found that the Port Fairy shark was of the order of 5 m (17 feet) in length and suggested that a mistake had been made in the original record, in 1870, of the shark's length.

Ellis and McCosker write that "the largest white sharks accurately measured range between 19 and 21 ft [about 5.8 to 6.4 m], and there are some questionable 23-footers [about 7 m] in the popular — but not the scientific — literature". Furthermore, they add that "these giants seem to disappear when a responsible observer approaches with a tape measure." (For more about legendary exaggerated shark measurements, see the submarine).

The largest specimen Ellis and McCosker endorse as reliably measured was 6.4 metres (21.3 ft) long, caught in Cuban waters in 1945 (though confident in their opinion, Ellis and McCosker note, however, that other experts have argued this individual might have been a few feet shorter). Click here to see a photo of this Cuban shark. There are more photos available to the public of this particular Cuban specimen. The photos and the story of the shark hunt were published in the Polk Voice article "A Shark to Remember: The Story of a Great White Shark" by writer Eduardo J. Echenique.

There have since been claims of larger great white sharks, but, as Ellis and McCosker note, verification is often lacking and these extraordinarily large great white sharks have, upon examination, all proved of average size. For example, a female said to be 7.13 metres (over 23 ft) was fished in Malta in 1987 by Alfredo Cutajar.

In their book, Ellis and McCosker agree this shark seemed to be larger than average, but they did not endorse the measurement. In the years since, experts eventually found reason to doubt the claim, due in no small part to conflicting accounts offered by Cutajar and others. A BBC photo analyst concluded that even "allowing for error ... the shark is concluded to be in the 18.3 ft [5.5 m] range and in no way approaches the 23 ft [7 m] reported by Abela." (as in original)

According to the Canadian Shark Research Centre, the largest accurately measured great white shark was a female caught in August 1988 at Prince Edward Island off the Canadian (North Atlantic) coast and measured 6.1 metres (20.3 ft). The shark was caught by David McKendrick, a local resident from Alberton, West Prince[citation needed].

The question of maximum weight is complicated by an unresolved question: when weighing a great white shark, does one account for the weight of the shark's recent meals? With a single bite, a great white can take in up to 14 kilograms (30 lb) of flesh, and can gorge on several hundred kilograms or pounds of food.

Ellis and McCosker write in regards to modern great white sharks that "it is likely that [great white] sharks can weigh as much as 2 tons", but also note that the largest recent scientifically measured examples weigh in at about 2 tonnes (1.75 short tons).

The largest great white shark recognized by the International Game Fish Association (IGFA) is one landed by Alf Dean in south Australian waters in 1959, weighing 1,208 kilograms (2,664 lb). Several larger great white sharks caught by anglers have since been verified, but were later disallowed from formal recognition by IGFA monitors for rules violations.

Adaptations

Great white sharks, like all other sharks, have an extra sense given by the Ampullae of Lorenzini, which enables them to detect the electromagnetic field emitted by the movement of living animals. Every time a living creature moves it generates an electrical field and great whites are so sensitive they can detect half a billionth of a volt. This is equivalent to detecting a flashlight battery from 1,600 kilometres (1,000 miles) away.

To more successfully hunt fast moving and agile prey such as sea lions, the poikilothermic great white shark has developed adaptations that allow it to maintain a body temperature warmer than the surrounding water. One of these adaptations is a "rete mirabile" (Latin for "wonderful net"). This close web like structure of veins and arteries, located along each lateral side of the shark conserves heat by warming the cooler arterial blood with the venous blood that has been warmed by the working muscles.

This keeps certain parts of the body running at temperatures up to 14° C[citation needed] above the surrounding water, while the heart and gills remain at sea-temperature. When conserving energy (a great white shark can go weeks between meals), the core body temperature can drop to match the surroundings. A great white shark's success in raising its core temperature is an example of gigantothermy. Therefore, the great white shark can be considered an endothermic poikilotherm, because its body temperature is not constant but is internally regulated.

Diet

Great white sharks primarily eat fish, smaller sharks, turtles, dolphins, whale carcasses and pinnipeds such as seals and sea lions. Great whites have also been known to eat objects that they are unable to digest. In great white sharks above 3.41 metres (11 ft, 2 in) a diet consisting of a higher proportion of mammals has been observed.

Behavior

A great white shark primarily uses its extra senses (i.e, electrosense and mechanosense) to locate prey from far off. Then, the shark uses smell and hearing to further verify that its target is food. At close range, the shark utilizes sight for the attack.

Great white sharks' reputation as ferocious predators is well-earned, yet they are not (as was once believed) indiscriminate "eating machines". They typically hunt using an "ambush" technique, taking their prey by surprise from below. Off Seal Island in South Africa studies have shown that the shark attacks most often in the morning, within 2 hours after sunrise. The reason for this is that it is hard to see a shark close to the bottom at this time. The success rate of attacks on average is 55% in the first 2 hours, it falls to 40% in late morning and after that the sharks stop hunting.

The ambush tactic, combined with the shark's ability to attain high speeds and its considerable mass, often cause great white sharks to breach (in a similar fashion to a whale breach) when attacking seals.[8] Other sharks which have been observed to breach the water are the thresher shark, shortfin mako, longfin mako, spinner shark, basking shark, blacktip shark, salmon shark, porbeagle shark and the copper shark.

The great white shark will often deliver a massive disabling bite and then back off to allow the prey to expire. This tactic allows the animal to avoid combat with dangerous prey, such as sea lions. It also has allowed occasional rescue of humans bitten by the animal, though it appears to attack humans mostly in error.

The great white shark is one of the only sharks known to regularly lift its head above the sea surface to gaze at other objects such as prey; this is known as "spy-hopping". This behaviour has also been seen in at least one group of blacktip reef sharks, but this might be a behaviour learned from interaction with humans (it is theorized that the shark may also be able to smell better this way, because smells travel through air faster than through water).

They are very curious animals, and can display a high degree of intelligence and personality when conditions permit (such as in the clear waters off of Isla Guadalupe, Mexico).

Reproduction

There is still a great deal that is unknown about great white shark behaviour, such as their mating habits. Birth has never been observed, but several pregnant females have been examined.

Great white sharks are ovoviviparous, the eggs developing in the female's uterus, hatching there and continuing to develop until they are born, at which point they are perfectly capable predators. The embryos can feed off unfecundated eggs. The delivery takes place in the period transitioning spring and summer.

The young, which number 8 or 9 (with a maximum of perhaps 14) for a single delivery, are about 1.5 metres (5 ft) long when born. Their teeth are provided with small side cusps. They grow rapidly, reaching 2 metres of length in the first year of life. Almost nothing, however, is known about how and where the great white mates. There is some evidence that points to the near-soporific effect resulting from a large feast (such as a whale carcass) possibly inducing mating.

A great white shark can reproduce when a male's length is around 3.8 metres (12 ft) and a female's length is around 4 to 4.8 metres (13.3 to 15.8 ft). Their lifespan has not been definitively established, though many sources estimate 30 to 40 years. It would not be unreasonable to expect such a large marine animal to live longer however.

Relationship to humans

Shark attacks

More than any documented attack, Steven Spielberg's 1975 film Jaws provided the great white shark with the image of a "man eater" in the public mind. While great white sharks have been responsible for fatalities in humans, they typically do not target humans as prey: for example, in the Mediterranean Sea there were 31 confirmed attacks against humans in the last two centuries, only a small number of them deadly.

Many incidents seem to be caused by the animals "test-biting" out of curiosity. Great white sharks are known to perform test-biting with buoys, flotsam, and other unfamiliar objects as well, and might grab a human or a surfboard with their mouth (their only tactile organ) in order to determine what kind of object it might be.

Other incidents seem to be cases of mistaken identity, in which a shark ambushes a bather or surfer, usually from below, believing the silhouette it sees on the surface is a seal. Many attacks occur in waters with low visibility, or other situations in which the shark's senses are impaired. It has been speculated that the species typically does not like the taste of humans, or at least that the taste is unfamiliar.

However some researchers have hypothesized that the reason the proportion of fatalities is low is not because sharks do not like human flesh, but because humans are often able to get out of the water after the shark's first bite.

In the 1980s John McCosker noted that divers who dived solo and were attacked by great whites were generally at least partially consumed, while divers who followed the buddy system were normally pulled out of the water by their colleagues before the shark could finish its attack.

Tricas and McCosker suggest that a standard attack modus operandi for great whites is to make an initial devastating attack on its prey, and then wait for the prey to weaken before going in to consume the ailing animal. A human's ability to get to land (or onto a boat) with the help of others is unusual for a great white's prey, and thus the attack is foiled.

Humans, in any case, are not healthy for great white sharks to eat because the sharks' digestion is too slow to cope with the human body's high ratio of bone to muscle and fat. Accordingly, in most recorded attacks, great whites have broken off contact after the first bite. Fatalities are usually caused by loss of blood from the initial limb injury rather than from critical organ loss or from whole consumption.

Biologist Douglas Long and Tyler B. write that the great white shark's "role as a menace is exaggerated; more people are killed in the U.S. each year by dogs than have been killed by great white sharks in the last 100 years." [10] However, such comments should be taken in context; interaction between humans and canines takes place far more regularly and in greater numbers than it does between humans and sharks.

Many "shark repellents" have been tested, some using scent, others using protective clothing, but to date the most effective is an electronic beacon (POD) worn by the diver/surfer that creates an electric field which disturbs the shark's sensitive electro-receptive sense organs, the ampullae of Lorenzini.

Great white sharks in captivity

All attempts to keep a great white shark in captivity prior to August 1981 lasted 11 days or less. However, that month a great white broke previous records by lasting 16 days in captivity at SeaWorld San Diego before being released into the wild.

In 1984, shortly before opening day, the Monterey Bay Aquarium in Monterey, California housed its first great white shark, which died after 10 days. In July 2003, Monterey researchers captured a small female and kept it in a large, netted pen off Malibu for five days, where they had the rare success of getting the shark to feed in captivity before it was released. It was not until September 2004 that the aquarium was the first to place a great white on long-term exhibit.

The young female, who was caught off the coast of Ventura, was kept in the aquarium's massive 1 million-gallon (3,800,000 litres) Outer Bay exhibit for 198 days before her successful release back to the wild in March 2005. She was tracked for 30 days after her early morning release. On the evening of August 31, 2006 the aquarium introduced a second shark to the Outer Bay exhibit.

The juvenile male, caught outside Santa Monica Bay on August 17 , had its first official meal in captivity (a large salmon steak) on September 8, 2006 and as of that date, the shark was estimated to be 1.72 metres (5 ft 8 in) and to weigh approximately 47 kilograms (104 lb). He was released on January 16, 2007 after 137 days in captivity.

Probably the most famous great white shark to be kept in captivity was a female named "Sandy", which in August 1980 became the first and only great white shark to be housed at the Steinhart Aquarium in San Francisco, California. She was returned to the wild because she would not eat anything given to her and constantly bumped against the walls.

Shark tourism and cage diving

Viewing sharks from the safety of a cage gives tourists an adrenaline rush and has become a booming industry. Common practice is to chum the water to draw in sharks for the tourists to view.

These practices have raised the fear that as a result of this form of tourism, sharks are becoming accustomed to people in their environment and beginning to associate human activity with food - a potentially dangerous situation. It is claimed that certain methods of chumming where the bait on a wire is drawn towards the divers in the cage, sometimes resulting in the shark striking the cage, exacerbate this problem. Other operators purposefully draw the bait away from the cage causing the shark to swim past the divers.

Shark cage-diving is when a group of tourists, or those who wish to study the sharks up close are lowered into the water beside a boat, protected by a steel cage. From this view point it is easier to view the sharks up close without the dangers (being bitten). Cage diving is most common off the coasts of Australia, South Africa, and Guadalupe Island off the coast of Baja California as it is here where great white sharks are most likely to be seen.

Companies respond that they are being made the scapegoats, as people try to find someone to blame for shark attacks on humans. Most point out that lightning tends to strike humans more often than sharks bite humans.Their position is that further research needs to be done before banning practices such as chumming which are said to alter sharks natural behaviour.

Conservation status

It is unclear how much a concurrent increase in fishing for great white sharks had to do with the decline of great white shark population from the 1970s to the present. No accurate numbers on population are available, but populations have clearly declined to a point at which the great white shark is now considered endangered. Their reproduction is slow, with sexual maturity occurring at about nine years of age, the population, therefore, can take a long time to rise.

In 2005, a tagged great white shark named "Nicole" was recorded swimming from South Africa to Australia and back, a 22,000 kilometre round trip. Researchers believe it may have undertaken this journey to mate, and hope studies such as this will produce more effective conservation measures.

The Convention on International Trade in Endangered Species (C.I.T.E.S.) has put the great white shark on its 'Appendix II' list of endangered species. The shark is targeted by fishermen for its jaws, teeth, and fins, and as a game fish. The great white shark, however, is rarely an object of commercial fishing, although its flesh is considered valuable. If casually captured (it happens for example in some tonnare in the Mediterranean), it is sold as smooth-hound shark.

From April 2007 great white sharks will be fully protected within 200 nautical miles of New Zealand and additionally from fishing by New Zealand-flagged boats outside this range.

Related species

Dental features and the extreme size of both the great white shark and the megalodon, Carcharodon megalodon, lead some scientists to believe they were closely related, however there is much doubt about this hypothesis and other scientists would place the megalodon and white shark as distant relatives - sharing the family Lamnidae but no closer relationship.

Megalodon is only known from its teeth, and may have reached sizes of 12 metres+ (40 ft) or more, considerably larger than even the largest great white sharks. From time to time it is suggested that megalodon might still exist. Megalodon teeth have been found from as recently as 10,000 to 12,000 years ago, though some have questioned the reliability of these estimates. However, while megalodon fossils are widespread and plentiful, no evidence has surfaced that the species is anything but extinct.

Other evidence suggests that the great white shark is more closely related to the mako shark than to the megalodon.

The true seals or earless seals are one of the three main groups of mammals within the seal suborder, Pinnipedia. All true seals are members of the family Phocidae. They are sometimes called crawling seals to distinguish them from the fur seals and sea lions of family Otariidae. Seals live in the oceans of both hemispheres and are mostly confined to polar, sub-polar, and temperate climes, with the exception of the more tropical monk seals.

Phocids are more highly specialized for aquatic life than otariids, although they still return to dry land or pack ice in order to breed and give birth. They lack external ears and have sleek, streamlined bodies. To further aid streamlining,

their nipples can be retracted, their testicles are internal, and the penis lies in an internal sheath. A smooth layer of blubber lies underneath the skin, and phocids are able to divert blood-flow to this layer to help control their temperature.

Their fore-flippers are used primarily for steering, while their hind flippers are bound to the pelvis in such a way that they cannot bring them under their body to walk on them. Phocids swim by sideways movements of their bodies, using their hind-flippers to their fullest effect.

They are more streamlined than fur seals and sea lions and can therefore swim more effectively over long distances. However, because they cannot turn their hind flippers downward, they are very clumsy on land, having to wriggle with their front flippers and abdominal muscles.

Phocid respiratory and circulatory systems are adapted to allow diving to considerable depths, and they can spend a long time underwater between breaths. Air is forced from the lungs during a dive and into the upper respiratory passages, where gases cannot easily be absorbed into the bloodstream. This helps protect the seal from the bends. The middle ear is also lined with blood sinuses that inflate during diving, helping to maintain a constant pressure.

True seals do not communicate by "barking" like otariids. Instead, they communicate by slapping the water and grunting.

Adult phocids vary from 1.17 meters in length and 45kg in weight, in the Ringed Seal, to 4.9 meters and 2,400kg in the Southern Elephant Seal.

Phocids have a reduced number of teeth compared with land-based members of the Carnivora, although they retain powerful canines. Some species lack molars altogether. The dental formula is:

2-3.1.4.0-2
1-2.1.4.0-2

Evolution
The earliest fossil phocids date from the mid-Miocene, 15 million years ago in the north Atlantic. Until recently, many researchers believed that phocids evolved separately from otariids and odobenids from otter-like animals, such as Potamotherium, which inhabited European fresh-water lakes. Recent evidence strongly suggests a monophyletic origin for all pinnipeds from a single ancestor, possibly Enaliarctos, most closely related to the bears.

Monk seals and elephant seals are believed to have first entered the Pacific through the open straits between North and South America, which closed only in the Pliocene. The various Antarctic species may have either used the same route, or travelled down the west coast of Africa.

While otariids are known for speed and maneuverability in the water, phocids are known for efficient, economical movement. This allows most phocids to make long foraging trips to exploit prey resources that are far from land, whereas otariids are tied to rich upwelling zones close to their breeding sites.

A pregnant female spends a long period of time foraging at sea, building up her fat reserves and then returns to the breeding site and uses her stored energy reserves to provide milk for her pup. The Common Seal, Phoca vitulina, displays a reproductive strategy similar to those of otariids in which the mother makes short foraging trips between nursing bouts.

Because a phocid mother's feeding grounds are often hundreds of kilometers from the breeding site, she must fast while she is lactating. This combination of fasting with lactation is one of the most unusual and extraordinary behaviors displayed by the Phocidae, because it requires the mother seal to provide large amounts of energy to her pup at a time when she herself is taking in no food (and often, no water) to replenish her stores.

Because they must continue to burn fat reserves to supply their own metabolic needs while they are feeding their pups, phocid seals have developed an extremely thick, fat-rich milk that allows them to provide their pups with a large amount of energy in as small a period of time as possible.

This allows the mother seal to maximize the efficiency of her energy transfer to the pup and then quickly return to sea to replenish her reserves. The length of lactation in phocids ranges from 28 days in the Northern Elephant Seal to just 3–5 days in the Hooded Seal.

The nursing period is ended by the mother, who departs to the sea and leaves her pup at the breeding site. Pups will continue to nurse if given the opportunity, and "milk stealers" that suckle from unrelated, sleeping females are not uncommon; this often results in the death of the pup whose mother the milk was stolen from, as any single female can only produce enough milk to provision one pup.

The pup's diet is so high-calorie that the pup builds up a large store of fat. Before the pup is ready to forage on its own, the mother abandons it, and it lives on its fat for weeks or months while it develops independence. Seals, like all marine mammals, need time to develop the oxygen stores, swimming muscles and neural pathways necessary for effective diving and foraging.

Seal pups typically eat no food and drink no water during the fast, although some polar species have been observed to eat snow. The post-weaning fast ranges from two weeks in the Hooded Seal to 9–12 weeks in the Northern Elephant Seal. The physiological and behavioral adaptations that allow phocid pups to endure these remarkable fasts, which are among the longest for any mammal, remain an area of active study and research.