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fish

Aquatic vertebrate that uses gills to obtain oxygen from fresh or seawater. There are three main groups: the bony fishes or Osteichthyes(goldfish, cod, tuna); the cartilaginous fishes or Chondrichthyes

(sharks, rays); and the jawless fishes or Agnatha (hagfishes,lampreys).

Fishes of some form are found in virtually every body of water inthe world except for the very salty water of the Dead Sea and someof the hot larval springs. Of the 30,000 fish species, approximately2,500 are freshwater.

Bony fishesThese constitute the majority of living fishes (about 20,000 species).

The skeleton is bone, movement is controlled by mobile fins, andthe body is usually covered with scales. The gills are covered by asingle flap. Many have a swim bladder with which the fish adjustsits buoyancy. Most lay eggs, sometimes in vast numbers; some codcan produce as many as 28 million. These are laid in the open sea,and probably no more than 28 of them will survive to becomeadults. Those species that produce small numbers ofeggs very oftenprotect them in nests, or brood them in their mouths. Some fishesare internally fertilized and retain eggs until hatched inside the

body, then giving birth to live young. Most bony fishes are ray-finned fishes, but a few, including lungfishes and coelacanths, arefleshy-finned.

Cartilaginous fishesThese are efficient hunters. There are fewer than 600 known speciesof sharks and rays. The skeleton is cartilage, the mouth is generallybeneath the head, the nose is large and sensitive, and there is a seriesof open gill slits along the neck region. They have no swimbladderand, in order to remain buoyant, must keep swimming. They may

lay eggs ('mermaid's purses') or bear live young. Some types ofcartilaginous fishes, such as sharks, retain the shape they hadmillions of years ago.

Jawless fishesJawless fish have a body plan like that of some of the earliestvertebrates that existed before true fishes with jaws evolved. Thereis no true backbone but a notochord. The lamprey attaches itself tothe fishes on which it feeds by a suckerlike rasping mouth.Hagfishes are entirely marine, very slimy, and feed on carrion and

injured fishes.

The world's largest fish is the whale sharkRhineodon typus, more

fish anatomy

fish market,Sarasota

frog-fish

grouper

hawkfish

tasselledscorpion fish

turkeyfish

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than 20 m/66 ft long; the smallest is the dwarf pygmy gobyPandaka pygmaea), 7.59.9 mm long. The study of fishes is calledichthyology.

Fish as foodThe nutrient composition of fish is similar to that of meat, exceptthat there are no obvious deposits of fat. Examples of fishcomparatively high in fat are salmon, mackerel, and herring. Whitefish such as cod, haddock, and whiting contain only 0.44% fat.Fish are good sources of B vitamins and iodine, and the fatty fishlivers are good sources of A and D vitamins. Calcium can beobtained from fish with soft skeletons, such as sardines. Roe andcaviar have a high protein content (2025%).

Physical features

The typical fish is streamlined. The skin usually contains pigmentcells (chromatophores), which enable the fish to change colour tomatch its surroundings, and mucous glands, which aid osmoticregulation (water and salt balance) and help protect the skin frombacteria by coating it with a layer of mucus. In most species,protection against abrasion is provided by scales. Oxygen isobtained from the water which is pumped via the mouth to the gills.The efficiency of the gaseous exchange system is increased using acounter current system which ensures that the least fully oxygenatedblood is opposite the most fully oxygenated water; at the same time,

carbon dioxide is lost to the water. The gills are usually covered andare ventilated by movements of the mouth and operculum (gillcover). Periodically the direction of the water current is reversed toclear the gills of detritus, a 'coughing reflex'.

The majority of fishes are predators, feeding on other fishes andinvertebrates. Most of them swallow their prey whole, the teethbeing used for grasping and orientating for swallowing. The form ofthe stomach varies enormously from the lampreys, which have

virtually no stomach, to the perch which have well-definedstomachs. The fin system of fishes is designed to provide stability inswimming with minimum loss of manoeuvrability; it is activated bythe muscles.

Smell is important in almost all species of fish, especially sharks.Many fishes show a well developed sense of taste, with taste budsnot only in their mouth cavities but also on other parts of theirbodies. Several species have barbels, whiskerlike structures aroundthe mouth covered with taste buds, which are used to search the

substrate. Most fishes can see quite well and many shallow waterfishes have colour vision, but some deep sea fishes and cave fishesare blind. Nearly all fishes have a lateral line system consisting of a

yellowdamselfish

fish:classification

Fishing:

DecliningStocks

Atlantic Cod

EndangeredSpecies HomePage

Fish FAQ

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series of small canals with pit organs that detect pressure. Thissystem, which is lost in higher vertebrates except for someamphibians, provides the fish with valuable information on watercurrents and pressure (depth). Some species have organs thatproduce an electrical field, which can be used to stun prey, inform

the fish of changes in its environment, and for communicationpurposes.

Osmotic regulationMarine fish live in a medium of greater concentration than theirbody fluids; this means that they tend to gain salts by diffusion andlose water by osmosis. To offset the loss by osmosis, they drink seawater, most of which their kidneys are able to retain the salt isexcreted, often through special cells in the gills. Freshwater fisheshave the opposite problem and have relatively large kidneys to

excrete the water entering their body by osmosis. To offset the lossof salts, freshwater fishes can obtain salts from their food; manyalso have salt-absorbing cells in their gills and mouths. They drinkvery little water and take in very little with their food. Severalspecies, such as the salmon, travel between sea water and freshwater; they therefore have to spend a short period in brackish waterto allow their bodies to adapt to the new conditions.

Did You Know?

The parasitic male anglerfish stays with his partner forlife literally. Once the tiny male is attached to the muchlarger female, his tissues fuse with hers, he loses hissenses of sight and smell, and receives his nourishmentfrom her bloodstream.

Did You Know?

Spurdog, alewife, twaite shad, jollytail, tadpole madtom,

bummalow, walleye pollock, wrestling halfbeak,mummichog, jolthead porgy, sweetlip emperor, andslippery dick are all common names for species of fish.

Did You Know?

The larvae of more than 50 species of fish living in theGreat Barrier Reef were found to achieve an averagemaximum swimming rate of 20.6 cm/8.1 in per second.

This is the equivalent of a human swimmer completingthe 100-m race in 3 seconds. The damselfish can cover50 body lengths per second and a soldierfish was timed

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Copyright Helicon Publishing Ltd 2000. All rights reserved.

averaging 56 cm/22 in per second over 10 minutes.

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