(Xiphophorus helleri), popular tropical fish of the live-bearer (q.v.) family Poeciliidae (order Atheriniformes). The swordtail is an elongated fish, growing to about 13 centimetres (5 inches) long and characterized, in the male, by a long, swordlike extension of the lower tail fin lobe. The fish is closely related to the platy (q.v.) and, like its relative, has been used extensively in genetic and medical research. Originally green with a red stripe on each side, the swordtail has been bred in many colour varieties.