Ray-finned fish own almost every living fish niche
Nearly ninety-nine percent of more than thirty thousand living fish species are ray-finned actinopterygians. Teleosts alone make up over half of all living vertebrates. From deep sea to caves, their fan-like fins—skin webs on thin bony rays—outmaneuver the fleshy paddles of lobe-finned cousins.
The name Actinopterygii joins Greek roots for ray and wing or fin. Light lepidotrichia support skin webbing that can reshape, tilt, and change area for high thrust per movement compared with sarcopterygian or shark fins. Most living forms are teleosts; their swim bladders usually bud dorsally from the gut for buoyancy, though bowfins and gars can still use the organ for breathing, and lineages such as arapaima reinvented air breathing. Teleosts also wear thin leptoid scales grown in concentric rings, unlike enamel-hard ganoid plates of some non-teleost actinopterygians.
Reproduction is diverse. Most species have separate sexes and broadcast eggs fertilised outside the body, then free-swimming larvae. Sequential hermaphroditism is common, usually protogyny—female first, then male—while protandry is rarer. Among oviparous teleost families, about seventy-nine percent offer no parental care; some form of care or live-bearing occurs in roughly twenty-one percent of four hundred twenty-two families surveyed, and viviparity appears in about six percent of living teleost species. The mangrove rivulus can self-fertilise as a simultaneous hermaphrodite, with males appearing mainly below nineteen degrees Celsius.
Living ray-fins split into Cladistia (bichirs and reedfish), Chondrostei (sturgeons and paddlefishes), and Neopterygii (holosteans plus teleosts). Teleosts exploded through the Mesozoic and Cenozoic until about ninety-six percent of living fish species belong to that infraclass. Whole-genome duplications mark major branches: roughly three hundred twenty million years ago in teleosts and around one hundred eighty million years ago in chondrosteans, with later repeats in salmonids and some cyprinids. The earliest known fossil actinopterygian, Andreolepis hedei, is about four hundred twenty million years old from Late Silurian rocks in Russia, Sweden, and Estonia.
Source: Actinopterygii
