teleost

Teleostei, commonly called teleosts, is a diverse infraclass within the ray-finned fishes (Actinopterygii) and constitutes the vast majority of living fishes. With around 26,000 described species, teleosts inhabit almost every aquatic environment, from freshwater rivers to the deepest oceans, and include forms as small as minute independent larvae and as large as migratory pelagic species.

A set of derived features helps distinguish teleosts from more primitive ray-finned fishes. They typically possess a mobile jaw mechanism, including an protrusible premaxilla and maxilla, which allows rapid suction feeding. Teleosts usually have a homocercal tail, a more flexible skull, and a highly adaptive buoyancy system via a gas bladder. The gas bladder is often connected to the gut by a pneumatic duct in many physostomes, while other teleosts (physoclists) regulate buoyancy without a duct. The skeleton is generally well ossified, and fin structures show considerable evolutionary diversification.

Evolutionarily, teleosts first appear in the fossil record in the late Triassic and underwent extensive diversification through the Cretaceous and Cenozoic. Their wide ecological breadth—predatory, herbivorous, planktivorous, nocturnal, diurnal—stems from feeding innovations, varied reproductive strategies, and adaptive radiation into new habitats.

Reproduction is diverse, but many species release eggs and sperm into the water (external fertilization), while others exhibit parental care, mouthbrooding, or nest guarding. Teleosts have significant ecological and economic importance, including key forage species, commercially important catches, and numerous model organisms such as the zebrafish (Danio rerio), widely used in medical and genetic research.