The term "fish" embraces several different types of aquatic animals, but about 96 percent of the species are represented by the bony fishes, which belong to the class Osteichthyes. Other types of fish include the jawless fish, such as lampreys, that feed by sucking, and the cartilaginous fish, a group that includes sharks, rays, and skates.
There are about 23,500 species of bony fishes, and it's believed that many more species, living at extreme depths, have yet to be discovered.
They range in size from the pygmy goby, about a half-inch in length, to the tuna and marlin, which may be as much as 15 feet long, and the ocean sunfish, which may weigh as much as a ton.
Bony fishes are cold-blooded animals that breath underwater, through gills. Other distinguishing features are a swim bladder, which is an internal sac that can be filled with air for buoyancy; scales made of thin, overlapping plates of bone that grow throughout life; and the external fertilization of eggs.