They have no swimbladder and, in order to remain buoyant, must keep swimming. 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 study of fishes is called ichthyology. 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. 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.