That means there are more fish species on Earth than all mammals, birds, reptiles, and amphibians combined. Over 24,000 fish have been named, and about 60 percent of these live in the salty seas. Being constrained to a life beneath the waves comes with its difficulties, but over the millennia, fish have found countless ways to adapt. About the only things that all fishes have in common are fins and gills. In fact, the variation among them is astounding.
Fishes come in three main body plans, all variations on a generally familiar fishy theme.
Most primitive are the jawless fishes, which include the hagfishes and lampreys. These fish have skeletons made entirely of cartilage and five to seven pairs of uncovered gill slits. There are more than 350 species of sharks roaming all corners of the oceans, and about 450 species of skates and rays. The largest group, by far, are the bony fishes, or teleosts. Bony fish have gills covered with a bone-like operculum, fins that can be folded or that change shape, and most have a swim bladder for buoyancy to help the fish maintain its position in the water column.