Killifish and live-bearers share a small-fish freshwater order
Cyprinodontiformes gathers mostly small freshwater ray-finned fishes, including popular aquarium killifish and live-bearers. They sit close to Atheriniformes and are sometimes discussed with them in older schemes. Adults range from an 8-millimetre killifish to a 34-centimetre four-eyed fish, and some species endure very warm or heavily polluted water.
Cyprinodontiformes is an order of ray-finned fish, mostly small and freshwater, that supplies aquarium keepers with killifish and live-bearers. The nickname toothcarps misleads: true carps sit in the superorder Ostariophysi, whereas these fish belong to the spiny-rayed Acanthopterygii and are close kin of the Atheriniformes, with which they are sometimes merged. The oldest possible fossil is ?Cyprinodon primulus, known only from scattered scales from the Late Paleocene of Argentina.
Reproduction splits the families three ways: egg-layers, ovoviviparous species whose young develop inside an eggshell within the mother, and viviparous ones whose young grow without that shell. The suborder Aplocheiloidei lays eggs exclusively, and live birth arose separately from egg-laying ancestors, the ovoviviparous route perhaps twice. Eschmeyer's 2025 catalogue lists families such as the egg-laying pupfishes, topminnows and African lampeyes alongside the largely viviparous splitfins and the livebearers, the Poeciliidae.
Most species have small mouths, large eyes, one dorsal fin and a rounded tail. Sizes run from the least killifish, adult at just 8 millimetres, to the cuatro ojos, Anableps dowei, at 34 centimetres. Some species survive in salty or very warm water, heavily polluted water, mineral-free rain pools soured by rotting leaves, or isolated spots where no other fish live.
Most are carnivores feeding near the oxygen-rich surface; in 1968 Scheel found ants in every gut he examined, and others report insects, worms and crustaceans. Pupfishes eat plants too, and the American flagfish is prized by aquarists for devouring algae. Killifish take pigment compounds such as carotenoids from pollen stuck to the insects they snatch off the surface, which colour-enhancing foods imitate in captivity.
Source: Cyprinodontiformes
