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For other uses, see Fish (disambiguation).
Fossil range: Mid Cambrian–Recent
Giant grouper swimming among schools of other fish
Head-on view of a red lionfish
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., gulpersand anglerfish). At 32,000 species, fish exhibit greater species diversity than any other group of vertebrates.
Fish are an important resource for humans worldwide, especially as food. 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 byrecreational 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.
Because the term "fish" is defined negatively, and excludes the tetrapods (i.e., the amphibians, reptiles, birds and mammals) which descend from within the same ancestry, it is paraphyletic, and is not considered a proper grouping insystematic biology. The traditional term pisces (also ichthyes) is considered a typological, but not a phylogenetic classification.
The earliest organisms that can be classified as fish were soft-bodied chordates that first appeared during the Cambrian period. Although they lacked a true spine, they possessed notochords which allowed them to be more agile than their invertebrate counterparts. Fish would continue to evolve through the Paleozoic era, diversifying into a wide variety of forms. Many fish of the Paleozoic developed external armor that protected them from predators. The first fish with jaws appeared in the Silurian period, after which many (such as sharks) became formidable marine predators rather than just the prey of arthropods.
3.6 Sensory and nervous system
3.6.1 Central nervous system
3.6.2 Sense organs
3.6.3 Capacity for pain
3.7 Muscular system
3.9 Reproductive system
4.1 Immune system
5.2 Habitat destruction
5.3 Exotic species
6 Importance to humans
6.1 Aquarium collecting
6.2 Economic importance
7.1 Shoal or school
7.2 Fish or fishes
8 See also
11 Further reading
12 External links
See also: evolution of fish
Outdated evolutionary view of continual gradation (click to animate)
Dunkleosteus was a gigantic, 10 meter (33 feet) long prehistoric fish.
Fish do not represent a monophyletic group, and therefore the "evolution of fish" is not studied as a single event.
Early fish from the fossil record are represented by a group of small, jawless, armored fish known as Ostracoderms. Jawless fish lineages are mostly extinct. An extant clade, the Lampreys may approximate ancient