The class Osteichthyes (literally "bony fish") gets its name from the bony skeleton and scales of its members. The group comprises nearly all living fish, with notable exceptions being sharks and other cartilaginous fish, and the primitive lampreys and their kin. Bony skeletons and scales are the primary features that differentiate these fish from other cartilaginous fish, whose skeleton is composed of cartilage and whose skin is leathery. Other important differences include the swim bladder, a lunglike, gas-filled organ that helps bony fish to regulate their buoyancy.
Osteichthyes is the most numerous and diverse group of vertebrates, occupying virtually all large bodies of water, from polar seas to hot undersea vents to land-locked lakes. Because of their numbers (more than 20,000 species), diversity, and range, the bony fish play a major role in virtually all marine and freshwater ecosystems . They range from the tiny seahorses to giant sunfish (weighing thousands of pounds) to the salmon on one's dinner plate.
Most of the bony fish—thirty-nine of the forty-two orders—are rayfinned fish; subclass Actinopterygii. The other three orders are fleshy-finned fish, members of the subclass Sacropterygii. Although much less numerous and diverse than the ray-fins, the fleshy finned fish are still interesting and important. In two orders, the fleshy finned fish have lungs instead of swim bladders, and can survive their ponds drying up by burrowing into the mud. The final order contains only one species: the coelacanth (pronounced SEE-low-kanth), an ancient species of fish once thought to be long extinct. Living coelacanths, virtually identical to its fossil relatives that lived 20 million years ago, were first found in 1938. This discovery was doubly important because the coelacanth is a close relative of the fish from which amphibians evolved, making it closely related to the ancestors of all terrestrial vertebrates.
Bond, Carl E. Biology of Fishes, 2nd ed. Fort Worth, TX: Saunders, 1997.
Moyle, Peter. B., and Joseph J. Cech, Jr. Fishes: An Introduction to Ichthyology, 4th ed. Englewood Cliffs, NJ: Prentice Hall, 1999.