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     Fish are an aquatic vertebrate animal, usually possessing gills in the adult stage and having limbs, when present, in the forms of fins. Unlike other commonly recognized groups of animals, the fishes are a diverse assemblage of groups that cannot be recognized by any defining trait (as mammals, for example, can be recognized by the presence of mammary glands or hair). Consequently, there are exceptions to the definition given here. Fishes are usually understood to include the jawless vertebrates such as the lamprey and hagfish; and the shark, ray, chimaera, lungfish, and bony fishes. The latter group includes the forms most commonly known as fish.Fishes constitute more than half the total number of known modern vertebrates. Scientists recognize an estimated 24,000 living species, compared with 21,500 extant (surviving) amphibians, reptiles, birds, and mammals. Unlike other vertebrates, new species of fishes continue to be discovered at a high rate and it is expected that the eventual number of recognized living species may exceed 28,000.























      In general, fishes are somewhat torpedo-shaped, with bodies that are moderately compressed at the sides and taper more markedly at the tail than at the head. The fundamental feature is the serially repeated set of vertebrae and segmented muscles, which enable the fish to propel itself by moving its body from side to side. Typically, the body carries a number of fins, which are membranes supported by rays or spines, that function in propulsion or orientation. One or more dorsal fins may be located along the centreline of the back. A caudal fin lies at the end of the tail





and is the primary organ for generating thrust in most species. One or more anal fins are situated on the ventral midline between the vent and the tail. The body has two pairs of lateral fins: the pectoral fins, which are usually located on the sides behind the gill openings, and the pelvic fins, which lie on the belly between the head and the vent. Great diversity exists in the shape and anatomical details of fishes, ranging from the snake-like eel to the globe-shaped puffer or greatly flattened fluke. Fins are often much modified or absent in accord with the diverse modes of life. Certain species of swamp eels lack most of the features by which fishes are commonly recognized, including gills, fins, and scales, and may even be primarily terrestrial. Some species of bony fishes lack eyes.
       The various species of fishes also show extreme variation in size. A recently discovered goby from the Indian Ocean reaches only 10 mm (about 0.5 in) in length, whereas the whale shark may reach 15 m (50 ft) and an estimated weight of 68,000 kg (150,000 lb).
       Fishes vary greatly in colouring as well, displaying a wide range of hues and patterns. In general the coloration is lighter on the belly than on the back, but in water the fish appears to be of a uniform shade because the only light comes from above. A number of tropical fishes, notably the butterfly fish, are brilliantly coloured and patterned. Their coloration may serve as a means of species recognition or, in the case of venomous species, as a warning to would-be predators. Many fishes have the ability to alter their colours markedly to blend with their surroundings. The flatfish has been observed changing colour to match a chequerboard pattern on the bottom of an aquarium.
Many species have specialized shapes and organs that aid them in hunting and feeding. Among such fishes are the lantern fish and other deep-water species, which have luminous organs to reveal or attract their prey. The anglerfish lies on the ocean bottom and presents a small, worm-like knob of flesh on the end of a long spine as a bait to other fishes.


























         The bodies of most fishes are covered with a layer of scales, which are bony or horny plates arranged in overlapping rows, with the free rear end of one scale overlapping the attached front end of the scale behind it. A thin epidermis usually covers the scales. In some species, such as the eel, the scales are minute. In others, such as the catfish, they are almost absent.








     Fishes have sometimes been classified according to the shape and characteristics of their scales. The most important types are ganoid scales, which are rhombic in shape and covered with an enamel-like layer; cycloid scales, which are almost round with smooth edges; and ctenoid scales, which are also round but have serrated, or comb-like, exposed edges. The epidermal layer of the body contains the pigment cells that give the fish its colour. Also in the epidermal layer are cells secreting a slippery mucus that covers the entire body.
























      The scaly skin covering the body of a fish comprises the dermal skeleton. The endoskeleton, or inner bony framework, of most modern fishes consists of a skull, containing jaws equipped with teeth; a vertebral column; ribs; a pectoral arch; and a series of interspinal bones that support the fins. In the ancient fishes, represented in modern times by such fishes as the sturgeon, skeletons are largely cartilaginous rather than bony.

































     Fish breathe by drinking. A gulp of water is forced under pressure from the mouth into a gill chamber on each side of the head. Gills themselves, located in gill clefts within the gill chambers, consist of fleshy, sheetlike filaments transected by extensions called lamellae. As water flows across the gills, the oxygen within them diffuses into blood circulating through vessels in the filaments




















      The digestive system of a fish usually consists of a mouth with rows of sharp, crushing, or brush-like teeth; a pharynx; an oesophagus; a stomach; and an intestine terminating in an anal vent. The several organs of the alimentary canal are not sharply differentiated in all species. All species, however, have a pancreas and a liver. The respiratory apparatus of jawed fishes consists of a series of slits, the gill clefts, which open from the pharynx



at the back of the throat to the gill chambers at each side of the back of the head. These chambers open to the water outside, but can be covered by a series of bones called the gill cover or operculum. Inside the gill chamber and in the gill clefts are the gills themselves, which take the form of thin sheets or filaments through which the blood circulates. When the fish takes in water and expels it through the gills, dissolved oxygen passes across the thin gill membranes into the blood, and carbon dioxide waste passes out of the blood into the water. A few species, however, such as the lungfish, can also breathe atmospheric air by means of a well-developed lung. Most bony fishes have an organ used in buoyancy control. Called the swim bladder, this precursor of the lung is a chamber that opens off the alimentary canal and fills with oxygen and nitrogen taken from the blood. The chief function of this organ is to adjust the fish to the varying water pressures at different depths so that the animal will have neither positive nor negative buoyancy. Thus the fish may maintain its depth without effort.
       The circulatory system in most fishes is simple, consisting of a two-chambered heart that forces blood forwards through the gills, then to the head, and from the head back to the rest of the body through a major artery situated beneath the spine. The rate of circulation is slower in fishes than in other vertebrates.
























     The chief muscles in the body of a fish are arranged along the sides of the trunk and tail. The larger mass runs along the back on each side of the spinal cord, and the smaller mass is located below it. Each muscle mass is composed of a series of interlocking segments. In ordinary swimming the successive contraction of the muscle segments from front to back on each side alternately gives the caudal fin a wavy motion. Small muscles control the motions of mouth, gills, fins, and eyes. A few types of fishes, such as the eel, swim by sinuous motions of their bodies. Certain others, among them the trunkfish, propel themselves by the action of their fins without much body motion.














































     The central nervous system of most fishes consists of a spinal cord and a brain that has a large cerebellum, a pair of optic lobes, a small cerebrum, and a medulla oblongata. The form and size of various parts of the brain vary markedly in different species. The eyes have almost spherical lenses with a flattened cornea. The entire lens of the eye is moved towards or away from the retina in order to focus at different distances. The eyes of some cave fishes that live in complete darkness are rudimentary or absent. Fishes smell by means of a pair of double nostrils leading into an olfactory pit, which is part of the smell organ. Many fishes detect chemical stimuli through sense organs or tentacles (barbels) around the mouth or on other parts of the body.
     Fish hear without the aid of external ears. Sound vibrations are transmitted through the bones of the skull to an internal ear containing three semicircular canals. This inner ear acts as an organ of equilibrium as well as an auditory organ. Fishes are also equipped with unique sensory organs called lateral lines. These organs consist of canals that run along the sides of the head and body and connect with the outside surface of the fish through small pores. The chief function of the lateral line is to sense extremely low-frequency vibrations, but in some species it can also detect weak electrical fields.
















      Water carries sound vibrations through small pores in a fish's skin and into the lateral line, an inner fluid-filled canal. Moving through the canal, the vibrations stimulate the hairs of sensory organs. The branching lateral nerve connects these sensory organs to the fish's brain, transmitting information about the flow of surrounding water and the movements of other organisms.







































     Most fishes are carnivorous and eat mainly other fishes but also worms, shellfish, and other kinds of water animals. The herbivorous fish have a diet of algae and other water plants, although they probably also eat fish. Some of the larger fishes, such as the whale shark, and also the flying fish and herring feed on plankton. Others are scavengers and live on waste products or the dead bodies of other animals found on the sea bed. Fish have a number of body organs that have been specially adapted for the capture of food. The razor-sharp teeth of barracudas, sharks, and certain piranhas are used to tear the flesh of their victims. Those fish that feed on plankton are known as filter-feeders as they have comb-like structures, gill rakers, that strain plankton from the water that is pumped through the gills. Electric fish, such as the electric eel, stun their prey with electric shocks, using their electricity-producing organs.






















     Fishes have various ways of producing young. Although methods of heterosexual reproduction are most common among fishes, some species are hermaphroditic-that is, individuals develop both ovaries and testes, either during separate life stages or simultaneously. Sexual parasitism is exhibited in some species of anglerfishes, in which the male permanently attaches himself to the body of the female, deriving nutrients from the female's circulatory system.
     Oviparous fishes are those that lay eggs that are fertilized outside the female's body. The female lays her eggs in the water and the male then covers them in sperm. In such species, development of the young is also external. Species that scatter eggs in open water often produce eggs in prodigious numbers. A single cod, for example, may produce up to 7 million eggs. Other egg-layers, such as the Pacific salmon, may undertake remarkable homing migrations associated with spawning activity. Parental care after hatching may be absent, or it may be elaborate, often involving the defence of a nest or territory. In the bowfin and some African species of cichlid, the young enter the mouth of a parent for protection when predators threaten.
     Viviparous fishes have internal fertilization and give birth to young in an advanced state of development. Viviparity has evolved many times among fishes and occurs in the shark, the coelacanth, and such popular aquarium fishes as the guppy and the mollie. These fish mate in pairs and copulate, fertilization of the eggs taking place inside the body of the female. Various mechanisms exist whereby nutrients are provided to the embryos, which may increase in size a thousandfold before birth. Ovoviviparity is exhibited in some species, in which the young hatch within the oviduct of the female and are thus born live.






















      Fishes occupy almost every conceivable aquatic habitat. The killifish is abundant in the world's highest large lake, South America's Lake Titicaca (3,810 m/12,500 ft above sea level); other species have been recorded from the bottom of the deepest lake, Lake Baikal (1,637 m/5,371 ft deep) in Russia, and from abyssal depths of the ocean (7,000 m/22,960 ft). In a Mexican hot spring, a killifish tolerates temperatures as high as 45° C (113° F), whereas the Antarctic icefish lives at about -2° C (29° F). The water does not freeze at this temperature because of its high salt content, and the fish does not freeze because its blood contains a form of biological antifreeze. Some fishes live in almost pure fresh water, while a killifish in Hispaniola tolerates salinity as high as four times that of the sea. Cave fishes may pass their lives in complete darkness, while fishes in desert marshes experience record levels of solar radiation. A group of annual fishes in South America survives periodic desiccation (drying out) by spending the dry season as dormant eggs, only to hatch and develop with the arrival of the next wet season.
     The greatest number of marine species is found in tropical waters, particularly in association with coral reefs. The greatest diversity of freshwater species occurs in the great lakes of Africa and in the streams of tropical rainforests, particularly in the Amazon Basin in South America.





















     Fish are one of the most important sources of animal protein for humans, and many fishes are used as food. Subsidiary uses of fish and fish products include the manufacture of nitrogenous fertilizers from fish and fish scraps, the extraction of fish-liver oils as one of the sources of vitamin D, and the manufacture of pet food. Fish scales are sometimes used in making artificial pearls. Isinglass, a form of gelatin, is prepared from the swim bladders of certain species, and glue can be made from fish offal

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