Over 33,000 fish species outnumber all other vertebrate classes
Fish are aquatic, gill-breathing vertebrates with skulls but no digit-bearing limbs, spanning jawless lineages and jawed forms. More than 33,000 living species make them the largest vertebrate group—more than amphibians, reptiles, birds, and mammals combined. Most are cold-blooded; some sharks and tunas keep warmer cores.
Fishlike creatures such as Haikouichthys, with a notochord and forward-facing eyes, show up about 530 million years ago in the Cambrian explosion, and conodonts follow late in that period. Early ostracoderms wore heavy bony plates against invertebrate predators. Armoured jawed placoderms like the giant Dunkleosteus appear in the Silurian alongside the first cartilaginous and bony fish, and the Devonian’s surge of placoderms, lobe-fins and early sharks earned it the name the age of fishes. Once the end-Devonian extinction removed the placoderm top predators, bony fish with swim bladders took over.
Of the living species, teleosts that can thrust their jaws forward make up about 96 percent. As of 2016 there were over 32,000 described bony fish, over 1,100 cartilaginous ones and more than 100 hagfish and lampreys. Size runs from the 16-metre whale shark to teleosts only 8 millimetres long, such as Paedocypris progenetica. Tuna and salmon can cover 10 to 20 body lengths a second, while eels and rays manage half a body length.
Species split almost evenly between some 15,200 freshwater and 14,800 marine kinds, with Indo-Pacific reefs and the Amazon, Congo and Mekong basins the richest. A cusk-eel holds the depth record at the bottom of the Puerto Rico Trench, Jonah’s icefish live under Antarctic ice at 79 degrees south, and desert pupfish tolerate springs as hot as 36 °C. Mudskippers forage on mudflats, and one undescribed Phreatobius catfish lives entirely in waterlogged leaf litter.
A fish heart pumps blood in a single loop through the gills, where blood runs opposite to the water flow for efficient countercurrent exchange. Silvery scales mirror the surrounding water as camouflage. Because fishes without the four-limbed animals descended from them form a paraphyletic group, formal classification has dropped the old class Pisces.
Source: Fish
