The words "reptile" and "bird" are precise in meaning, for they denote a member of a particular class of vertebrate animals. The word "fish," however, although extremely useful, is not specifically precise, for there is no class "Fishes." The animals usually referred to as fishes belong to several classes. All are aquatic and are constructed on a similar pattern, but there are a number of important differences between the classes.
The class Elasmobranchii contains the sharks and rys whose skeletons are made up completely of cartilage. The class Actinopterygii is by far the largest of the "fishy" classes with more than 20,000 species. The skeleton is usually made of true bone and the fins are membranous and supported by bony rays. The members of this class are thus often called the ray-finned fishes. They include the herring, carp, sea-horse, eel, and almost all other common fishes. The third class of living fishes is the Crossopterygii. Members of this class have true bony skeletons but their fins have fleshy lobes at the base. There are only a few living species in this class - most of its members died out long ago after giving rise to the ancestors of the land vertebrates. The living lung-fishes are very specialised survivors of this class but the coelacanth seems to have remained almost unchanged for millions of years. A few extinct classes are also usually grouped with the fishes.
The lampreys and hagfishes, although very different from all the fishes mentioned so far, are frequently dealt with with the fishes. They are the survivors of a very ancient group of vertebrate animals called the Agnatha, which literally means "without jaws," for these animals have no jaws - food is sucked into the mouth and swallowed. These animals also have no paired fins. Young lampreys show some similarities to the protochordates from which the vertebrates are believed to have arisen.
The food of fishes falls into 3 main classes - small floating organisms, other fishes, and bottom-living creatures such as mollusks and starfishes. Each species of fish keeps more or less to one type of diet and the mouth and teeth are well adapted to this diet.
Fishes feeding on small floating organisms usually have their mouths at the very front of the head and make darting movements at the prey which is gulped down with the water. Teeth in these fishes are usually poorly developed.
Those fishes which feed on otherr fishes usually have well developed teeth - not only on the jaw bones, but on the roof of the mouth and on other bones as well. The mouth is again usually at the front of the head.
Bottom-feeders normally have the mouth directed to the underside of the body. Their teeth are more or less plate-like and well suited to their job of crushing mollusks.
Lampreys feed mainly on other fishes. Although they have no jaws, they have a toothed tongue with which they rasp away the flesh of their prey. The food is then sucked into the mouth.
If all the fish eggs were to survive, the sea would literally be jam-packed with fishes. A female cod may contain as many as 9 million eggs, halibut more than 2 million, herring 30 to 50 thousand. Of course, the sea could not support such a population. Many of the eggs are eaten by fishes, often by fishes of the same species, and many are not even fertilized. The chances of an egg or of a young fish surviving are very low so that the production of so many eggs is necessary for the numbers to be maintained.
Not all the fishes oriduce as many eggs as those quoted above. As a general rule, the nmbers depend on the degree of care that the eggs and young receive form the parent fishes. Female sticklebacks lay from 50 to 100 eggs only, in an elaborate nest usually constructed of seaweed in marine species, or algal threads and water weeds in freshwater species. The nest is built by the male and he zealously guards it and its immediate surroundings. Even when the eggs hatch he guards the young just as carefullly until they are able to fend themselves.
The reproductive habits of fishes vary enormously. Many, such as herring and mackerel, assemble in vast shoals prior to spawning, while salmon form pairs. The act of pairing may be preceded by an elaborate courtship display, prior to which the male often becomes brilliantly colored.
Many shark-like fishes bear their young alive; so do a few bony fishes, for example, guppies, surf fishes and live-bearers).
Among those fishes that do look after the eggs and young, one or both parents may be involved. In the African lake fish (Tilapia) the female usually incubates and guards the eggs in her mouth. For a time after the young have hatched they are taken back into her mouth periodically, especially at the approach of danger. They are protected in this way for almost a week. In related species the male alone, or male and female in turn take the eggs and young into their mouths.