Fish are a paraphyletic group of organisms
that consist of all gill-bearing aquatic vertebrate
(or 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). At 32,000
species, fish exhibit greater species diversity
than any other class of vertebrates.
Fish, especially as food, are an important resource
worldwide. Commercial and subsistence fishers hunt
fish in wild fisheries (see fishing) or farm them
in ponds or in cages in the ocean (see aquaculture).
They are also caught by recreational fishers, kept
as pets, raised by fishkeepers, and exhibited in
public aquaria. Fish have had a role in culture
through the ages, serving as deities, religious
symbols, and as the subjects of art, books and movies.