Fish
| Fish | |
|---|---|
.jpg) | |
| Bala shark, a bony fish | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Olfactores |
| Subphylum: | Vertebrata |
| Groups included | |
| |
| Cladistically included but traditionally excluded taxa | |
A fish is an aquatic, anamniotic, gill-bearing vertebrate animal with a tough cranium to protect the brain, but lacking limbs with digits. Fish can be grouped into the more basal jawless fish and the more common jawed fish, the latter including all living cartilaginous and bony fish, as well as the extinct placoderms and acanthodians. In a break from the long tradition of grouping all fish into a single class (Pisces), modern phylogenetics views fish as a paraphyletic group that includes all vertebrates except tetrapods. In English, the plural of "fish" is fish when referring to individuals and fishes when referring to species.
Most fish are cold-blooded, their body temperature varying with the surrounding water, though some large, active swimmers like the white shark and tuna can maintain a higher core temperature. Many fish can communicate acoustically with each other, such as during courtship displays. The study of fish is known as ichthyology.
There are over 33,000 extant species of fish, easily the largest group of vertebrates and more than all species of the other traditional classes, namely amphibians, reptiles, birds, and mammals, combined. Most fish belong to the class Actinopterygii, the ray-finned fishes, which accounts for approximately half of all living vertebrates.
The earliest fish appeared during the Cambrian as small filter feeders; they continued to evolve through the Paleozoic, diversifying into many forms. The earliest fish with dedicated respiratory gills and paired fins, the ostracoderms, had heavy bony plates that served as protective exoskeletons against invertebrate predators. The first fish with jaws, the placoderms, appeared in the Silurian and greatly diversified during the Devonian, the "Age of Fishes".
Bony fish, distinguished by the presence of swim bladders and later ossified endoskeletons, emerged as the dominant group of fish after the end-Devonian extinction wiped out the apex predators, the placoderms. Bony fish are further divided into lobe-finned and ray-finned fishes. About 96% of all living fish species today are teleosts- a crown group of ray-finned fish that can protrude their jaws. The tetrapods, a mostly terrestrial clade of vertebrates that have dominated the top trophic levels in both aquatic and terrestrial ecosystems since the Late Paleozoic, evolved from lobe-finned fish during the Carboniferous, developing air-breathing lungs homologous to swim bladders.
Fish have been an important natural resource for humans since prehistoric times, especially as food. Commercial and subsistence fishers harvest fish in wild fisheries or farm them in ponds or breeding cages in the ocean. Fish are caught for recreation or raised by fishkeepers as ornaments for private and public exhibition in aquaria and garden ponds. Fish have had a role in human culture through the ages, serving as deities, religious symbols, and as the subjects of art, books and movies.
Etymology
The word fish is inherited from Proto-Germanic, and is related to German Fisch, the Latin piscis, and Old Irish íasc, though the exact root is unknown; some authorities reconstruct a Proto-Indo-European root *peysk-, attested only in Italic, Celtic, and Germanic.[1][2][3][4]
Though often used interchangeably, in biology fish and fishes have different meanings. Fish is used as a singular noun, or as a plural to describe multiple individuals from a single species. Fishes is used to describe different species or species groups.[5][6][7]
Evolution
Fossil history
About 530 million years ago during the Cambrian explosion, fishlike animals with a notochord and eyes at the front of the body, such as Haikouichthys, appear in the fossil record.[8] During the late Cambrian, other jawless forms such as conodonts appear.[9][10]
Jawed vertebrates appear in the Silurian, with giant armoured placoderms such as Dunkleosteus.[11] Jawed fish, too, appeared during the Silurian:[12] the cartilaginous Chondrichthyes[13][14] and the bony Osteichthyes.[15]
During the Devonian, fish diversity greatly increased, including among the placoderms, lobe-finned fishes, and early sharks, earning the Devonian the epithet "the age of fishes".[16][17]
Phylogeny
Fishes are a paraphyletic group, since any clade containing all jawed fish (Gnathostomata) or all bony fish (Osteichthyes) also contains the clade of tetrapods (four-limbed vertebrates, mostly terrestrial), which are usually not considered fish.[18][19] Some tetrapods, such as cetaceans and ichthyosaurs, have secondarily acquired a fish-like body shape through convergent evolution.[20] On the other hand, Fishes of the World comments that "it is increasingly widely accepted that tetrapods, including ourselves, are simply modified bony fishes, and so we are comfortable with using the taxon Osteichthyes as a clade, which now includes all tetrapods".[19]
The biodiversity of extant fish is unevenly distributed among the various groups; teleosts, bony fishes able to protrude their jaws, make up 96% of fish species.[21]