(species Xiphophorus maculatus), popular tropical aquarium fish of the live-bearer family, Poeciliidae (order Atheriniformes). The platy is a compact fish, about 5 cm (2 inches) long and extremely variable in colour. It has been bred in many attractive colour varieties, and, like the related swordtail (Xiphophorus helleri) with which it has been crossed, has been used in a variety of genetic and medical studies.