Fish live in the water and breathe by absorbing oxygen through their gills. Fish are cold-blooded animals, so their internal body temperature is influenced by the environment. Many fish have scales and use their fins to swim. Fish have a spine but do not have external ears or eyelids. Fish also have air bladders, which keep them afloat.
A fish's gills are special organs which contain thousands of small blood vessels called capillaries. The gills constantly filter oxygen out of water and into the fish's bloodstream. Gills are also useful for excreting waste from the fish's bloodstream. Some fish, such as sharks, have multiple gill openings.
Fish have sharp or smooth scales that protect the fish from injury. Fish create a mucous covering over the scales, which helps protect against infection by trapping bacteria and keeping it from entering the fish's body. This mucous also reduces friction, which helps the fish easily swim through the water.
According to the International Union for Conservation for Nature, about 800 species of fish live in the United States and there are 30,000 species worldwide. Scientists believe that thousands of fish species have not been identified. Fish are a more diverse species than any other group of vertebraes.