A fish is an aquatic, craniate, gill-bearing animal that lacks limbs with digits. This definition includes living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups. Fish are vertebrates that live in water and breathe using special organs called gills. They are poikilothermic, aquatic chordates with appendages developed as fins, and their chief respiratory organs are gills. Fish are found in both fresh and salt waters, and there are approximately 34,000 species of vertebrate animals that fall under the term "fish".
The term "fish" is used to refer to diverse aquatic organisms, such as lampreys, sharks, coelacanths, and ray-finned fishes. However, it is not a taxonomic group that would be used in a phylogenetic classification scheme, as "vertebrates" or "hominids" is, because phylogenetic taxonomic groups must be clades. A clade is a group that includes all the descendants of a common ancestor and that ancestor, and all the different organisms that are considered fish do not form a clade. This is because the lobe-finned lineage includes both the lobe-finned fish and four-legged vertebrates, like frogs, dinosaurs, bats, and humans, which are nested within a bunch of fish on the tree of life, causing the fish not to form a clade.
