Fish are the gill-beirin aquatic craniate ainimals that lack limms wi deegits. Thay form a sister group tae the tunicates, thegither formin the olfactores. Includit in this defineetion are the leevin hagfish, lampers, an cartilaginous an banie fish as weel as various extinct relatit groups. Tetrapods emerged athin lobe-finned fishes, sae cladistically thay are fish as weel. Houiver, tradeetionally fish are rendered paraphyletic bi excludin the tetrapods (i.e., the amphibians, reptiles, birds an mammals). Acause in this manner the term "fish" is defined negatively as a paraphyletic group, it is nae conseedert a formal taxonomic groupin in seestematic biology. The tradeetional term pisces (an aa ichthyes) is conseedert a teepological, but nae a phylogenetic clessification.
The earliest organisms that can be clessifee'd as fish war saft-bodied chordates that first appeared in the Cambrian period. Awtho thay lacked a true spine, thay possessed notochords that alloued them tae be mair agile nor thair invertebrate coonterpairts. Fish wad conteena tae evolve throu the Paleozoic era, diversifeein intae a wide variety o forms. Mony fish o the Paleozoic developed external airmour that pertectit them frae predators. The first fish wi jaws appeared in the Silurian period, efter that mony (sic as shairks) becam formidable marine predators raither than juist the prey o arthropods.
Maist fish are ectothermic ("cauld-bluidit"), allouin thair bouk temperaturs tae vary as ambient temperaturs cheenge, tho some o the lairge active soummers lik white shark an tuna can hauld a heicher core temperatur. Fish are abundant in maist bouks o watter. Thay can be foond in nearly aw aquatic environments, frae heich moontain streams (e.g., Salvelinus an gudgeon) tae the abyssal an even hadal deepths o the deepest oceans (e.g., gulpers an anglerfish). Wi 33,600 descrived species, fish exhibit greater species diversity than ony ither group o vertebrates.
Fish are an important resoorce for humans warldwide, especially as fuid. Commercial an subsistence fishers hunt fish in wild fisheries (see fishin) or ferm them in ponds or in cages in the ocean (see aquacultur). Thay are an aa catcht bi recreautional fishers, kept as pets, raised bi fishkeepers, an exhibitit in public aquaria. Fish hae haed a role in cultur throu the ages, servin as deities, releegious seembols, an as the subjects o airt, beuks an films.