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A fish is a
water-dwelling
vertebrate with
gills that doesn't change form, as amphibians do, during
its life. Most are
cold-blooded, though some (such as some species of tuna and
shark) are warm-blooded. There are over 29,000 species of
fish, making them the most diverse group of vertebrates.
Taxonomically, fish are a paraphyletic group whose exact
relationships are much debated; a common division is into
the jawless fish (class
Agnatha, 75 species including
lampreys and
hagfish), the cartilaginous fish (class Chondrichthyes,
800 species including sharks and rays), with the remainder
classed as bony fish (class
Osteichthyes).
Fish come in different sizes, from the 16 m (51 ft) whale
shark to a 8 mm (just over Ľ of an inch) long stout
infantfish. Many types of aquatic animals named "fish" are
not true fish, and in the case of animals such as jellyfish
and cuttlefish, are not even vertebrates. Other marine
creatures that have in the past been considered fish, like
dolphins, are actually mammals.
Although most fish are exclusively aquatic and
cold-blooded, there are exceptions to both cases. Fish from
a number of different groups have evolved the capacity to
live out of the water for extended periods of time. Of these
amphibious fish some such as the mudskipper can live and move about on land for up to
several days. Also, certain species of fish maintain
elevated body temperatures to varying degrees. Endothermic
teleosts (bony fishes) are all in the suborder
Scombroidei and include the billfishes, tunas, and one
species of "primitive" mackerel (Gasterochisma melampus).
All sharks in the family Lamnidae – shortfin mako, long fin
mako, white, porbeagle, and salmon shark – are known to have
the capacity for endothermy, and evidence suggests the trait
exists in family Alopiidae (thresher sharks). The degree of
endothermy varies from the billfish, which warm only their
eyes and brain, to bluefin tuna and porbeagle sharks who maintain body
temperatures elevated in excess of 20 °C above ambient water
temperatures. See also
gigantothermy. Endothermy, though metabolically costly,
is thought to provide advantages such as increased
contractile force of muscles, higher rates of central
nervous system processing, and higher rates of digestion.
Fish are an important source of food in many cultures.
Other water-dwelling animals such as mollusks, crustaceans,
and shellfish are often called "fish" when used as food. For
more details, see
Fish food.
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A fish is a
water-dwelling
vertebrate with
gills that doesn't change form, as amphibians do, during
its life. Most are
cold-blooded, though some (such as some species of tuna and
shark) are warm-blooded. There are over 29,000 species of
fish, making them the most diverse group of vertebrates.
Taxonomically, fish are a paraphyletic group whose exact
relationships are much debated; a common division is into
the jawless fish (class