Aquatic vertebrate that uses gills to obtain oxygen from fresh or sea water. 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 in the world except for the very salty water of the Dead Sea and some of the hot larval springs. Of the 30,000 fish species, approximately 2,500 are freshwater.
Bony fishes These constitute the majority of living fishes (about 20,000 species). The skeleton is bone, movement is controlled by mobile fins, and the body is usually covered with scales. The gills are covered by a single flap. Many have a swim bladder with which the fish adjusts its buoyancy. Most lay eggs, sometimes in vast numbers; some cod can produce as many as 28 million. These are laid in the open sea, and probably no more than 28 of them will survive to become adults. Those species that produce small numbers of eggs very often protect them in nests, or brood them in their mouths. Some fishes are 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, are fleshy-finned.
Cartilaginous fishes These are efficient hunters. There are fewer than 600 known species of sharks and rays. The skeleton is cartilage, the mouth is generally beneath the head, the nose is large and sensitive, and there is a series of open gill slits along the neck region. They have no swimbladder and, in order to remain buoyant, must keep swimming. They may lay eggs (‘mermaid's purses’) or bear live young. Some types of cartilaginous fishes, such as sharks, retain the shape they had millions of years ago.
Jawless fishes Jawless fish have a body plan like that of some of the earliest vertebrates that existed before true fishes with jaws evolved. There is no true backbone but a notochord. The lamprey attaches itself to the 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 shark Rhineodon typus, more than 20 m/66 ft long; the smallest is the dwarf pygmy goby Pandaka pygmaea), 7.5–9.9 mm long. The study of fishes is called ichthyology.
Fish as food The nutrient composition of fish is similar to that of meat, except that there are no obvious deposits of fat. Examples of fish comparatively high in fat are salmon, mackerel, and herring. White fish such as cod, haddock, and whiting contain only 0.4–4% fat. Fish are good sources of B vitamins and iodine, and the fatty fish livers are good sources of A and D vitamins. Calcium can be obtained from fish with soft skeletons, such as sardines. Roe and caviar have a high protein content (20–25%).
Physical features The typical fish is streamlined. The skin usually contains pigment cells (chromatophores), which enable the fish to change colour to match its surroundings, and mucous glands, which aid osmotic regulation (water and salt balance) and help protect the skin from bacteria by coating it with a layer of mucus. In most species, protection against abrasion is provided by scales. Oxygen is obtained from the water which is pumped via the mouth to the gills. The efficiency of the gaseous exchange system is increased using a counter current system which ensures that the least fully oxygenated blood is opposite the most fully oxygenated water; at the same time, carbon dioxide is lost to the water. The gills are usually covered and are ventilated by movements of the mouth and operculum (gill cover). Periodically the direction of the water current is reversed to clear the gills of detritus, a ‘coughing reflex’.
The majority of fishes are predators, feeding on other fishes and invertebrates. Most of them swallow their prey whole, the teeth being used for grasping and orientating for swallowing. The form of the stomach varies enormously from the lampreys, which have virtually no stomach, to the perch which have well-defined stomachs. The fin system of fishes is designed to provide stability in swimming with minimum loss of manoeuvrability; it is activated by the muscles.
Smell is important in almost all species of fish, especially sharks. Many fishes show a well developed sense of taste, with taste buds not only in their mouth cavities but also on other parts of their bodies. Several species have barbels, whiskerlike structures around the mouth covered with taste buds, which are used to search the substrate. Most fishes can see quite well and many shallow water fishes have colour vision, but some deep sea fishes and cave fishes are blind. Nearly all fishes have a lateral line system consisting of a series of small canals with pit organs that detect pressure. This system, which is lost in higher vertebrates except for some amphibians, provides the fish with valuable information on water currents and pressure (depth). Some species have organs that produce an electrical field, which can be used to stun prey, inform the fish of changes in its environment, and for communication purposes.
Osmotic regulation Marine fish live in a medium of greater concentration than their body fluids; this means that they tend to gain salts by diffusion and lose water by osmosis. To offset the loss by osmosis, they drink sea water, most of which their kidneys are able to retain – the salt is excreted, often through special cells in the gills. Freshwater fishes have the opposite problem and have relatively large kidneys to excrete the water entering their body by osmosis. To offset the loss of salts, freshwater fishes can obtain salts from their food; many also have salt-absorbing cells in their gills and mouths. They drink very little water and take in very little with their food. Several species, such as the salmon, travel between sea water and fresh water; they therefore have to spend a short period in brackish water to allow their bodies to adapt to the new conditions.
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