fish

fish are vertebrate animals that live in almost every part of the ocean, from the surface to the bottom of the deepest trenches, and even at the very edge of the sea. By 2004 over 20,000 species, including the 200 or so edible species, had been described, Since about 100 new species are being described each year, the total number of fishes may exceed 30,000 species.

About 60% of all vertebrate animals (animals with backbones) are fishes. About half the described species are marine, and approximately 75% of them live in shallow coastal waters. The types range from hagfish (Myxine spp.) to lungfish (Dipnoi), but here the discussion will centre on the bony fishes, or teleosts, which are by far the most diverse and species-rich group of fishes. They are distinct from the cartilaginous fishes, i.e. sharks and their relatives, by having bony rather than cartilaginous skeletons, gills covered with a flap, and mouths that are usually on the front of the head. The mouths of some fishes are armed with teeth that are used to rasp soft tissue, grind up molluscs, or scrape algae off rocks. Others, such as the seahorses, have no teeth and have tubular-shaped mouths adapted to suck up individual plankton.

The rich diversity of fishes is partly the result of their having adopted a great variety of ways of feeding. Their basic body form is spindle shaped, with dorsal fins on the back, two pairs of lateral fins, the pectorals and the pelvics, a ventral anal fin, and a large tail, or caudal, fin that is usually symmetrical. The fins are composed of fine bones, or fin rays that normally are webbed. These fin rays can be developed into long sensory structures or hard spiny structures that may be armed with poison glands. In remoras (family Echeneidae) the dorsal fin has been modified into a sucker. The spindle shape gives a good hydrodynamic shape that slides through the water with minimum resistance when the posterior region of the body, and the tail, beats from side to side. However, this basic body shape has been greatly modified in different families of fish, especially in those families that live around the seabed. Eels have lost most of their fins and developed long sinuous bodies and a serpentine mode of movement, ideal for moving in and out of crannies in, reefs, but less effective for swimming in midwater. Flatfishes (order Pleuronectiformes) have flattened bodies, with either the left side or the right side becoming the lower surface, and during development the head rotates 90°, whereas scorpion fishes (order Scorpaeniformes) have bodies that are flattened dorsoventrally. Boxfishes (family Ostraciidae) have inflexible armoured bodies, and so their mode of swimming is by sculling with their fins. Mudskippers (Periophthalmus spp.) that inhabit the fringes of mangrove swamps use their pectoral fins to climb up out of the water.

The coloration of fishes is almost as diverse is their habits. Some, like flatfishes, can change their colour at will, either to blend in with different backgrounds or to flash warnings to would-be predators. In many shallow coral reef species, males display bright colours to guard their territories, but generally fishes use their coloration as camouflage. The commonest colour pattern in the fishes that swim in midwater is a counter-shading, with dark backs and pale bellies and flanks that may be banded with a disruptive pattern, often silvery or, in deep-sea species, lined with light organs. The brightness of light changes with depth, and its colour also changes—red light being absorbed very quickly—so the range of colours used by fish is restricted. Also, fish see only monochromatic blue-green light, which is the colour of light that penetrates furthest in water. In very deep water many fish species do not have functional eyes, since that there is almost nothing to see in the permanent darkness below about 1,000 metres (3,250 ft).

Another important sensory feature of fishes is their lateral line system. This is a chain of sense organs, similar to those in our ears, that can either be open to the water or semi-enclosed. With this system the fishes feel the water, sensing currents and the low-frequency vibrations transmitted by the movements of other animals. Many deep-sea fishes have long filamentous tails that, by extending the length of the lateral line organ, enable them to feel the direction from which any movements are coming.

Fishes' blood is about half as salty as sea water—which is why thirsty shipwrecked mariners can safely drink it to slake their thirst—so fish continually have to get rid of salt from their bodies across their gills. Although this helps to make fishes less dense, many still have systems for adjusting the density of their bodies to be much the same as the sea water, so when they stop swimming they do not sink. Above the gut of many fishes is a swim-bladder that is filled with gas. In some fishes this is filled by gulping in air at the surface, but the vast majority never approach the sea surface, so the swim-bladder is filled by a special gland that extracts dissolved gases from the blood. Some fishes that swim continuously, like tunas and marlin, have no swim-bladder. Instead, they have rigid pectoral fins that act like hydroplanes to generate lift as they swim. Swim-bladders do not occur in many deep-sea fishes, because the greater the hydrostatic pressure (i.e. depth) the greater the energy required to fill them, so at depths below about 500 metres (1,625 ft) it becomes physiologically too expensive. In these fishes either the swim-bladder is filled with oily fats, or it disappears. Deep-sea fishes tend to have very watery tissues, and their bones contain very little calcium, which reduces their density. However, some still retain gas-filled swim-bladders in very deep water which are connected by bones to sensory organs, and function as hearing organs. They also often have drumming muscles attached to them for the fish to produce sounds for communication. During the Cold War when hydrophones were deployed in deep water to listen for the movements of submarines, it was found that the deep ocean is quite noisy, especially during the breeding season for deep-sea fishes and when whales migrate.

See also fisheries.

Bibliography

Bone, Q.,, Marshal, N. B.,, and and Blaxter, J. H. S. , The Biology of Fishes (1995).www.fishbase.org/home.htm

M. V. Angel