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.