A fish is any member of a paraphyletic group of organisms that consist of all
gill-bearing aquatic craniate animals that lack limbs with digits. Included in
this definition are the living hagfish, lampreys, and cartilaginous and bony
fish, as well as various extinct related groups. Most fish are ectothermic
("cold-blooded"), allowing their body temperatures to vary as ambient temperatures change,
though some of the large active swimmers like white shark and tuna can hold a higher
core temperature. Fish are abundant in most bodies of water. They can be found in
nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon)
to the abyssal and even hadal depths of the deepest oceans (e.g., gulpers and anglerfish).
With 33,100 described species, fish exhibit greater species diversity than any other
group of vertebrates.