Class Osteichthyes (the bony fish) is the largest class of vertebrates with over 20,000 species. The total numbers of fishes exceed that of all other kinds of vertebrates combined. This is not surprising considering four fifths of the earth's surface is covered with water. Bony fish have a skeleton much stiffer than the cartilaginous fish because it is reinforced by calcium salts. Bony fish have excellent smell like cartilaginous fish, but unlike the other class of fish, bony fish also have acute eyesight.

Bony fish also have special adaptations that allow them to remain buoyant. A special organ called a swim bladder housed under the bony skeleton is a gas filled chamber that allows the bony fish to remain floating in the water. Another special adaptation is the operculum, a bone on each side of the fish that covers the chambers housing the gills. A bony fish is able to breathe without swimming simply by moving the operculum. Other hallmarks of these fish are paired fins, many teeth, dermal scales in the skin (in most species), and numerous vertebrae. Most bony fish are ray-finned fish, meaning that they have thin, flexible skeletal rays. Lobe-finned fish, the other type of bony fish, have muscular fins supported by bones. Only one species of lobe-finned fish, the coelacanth, still lives.

Key features of Osteichthyes