Heterandria

Heterandria is a genus of livebearing fishes within the family Poeciliidae. Most species occur in Guatemala and its surroundings, particularly Mexico, but the midget livebearer (H. formosa) comes from the southeastern United States.

Though many Poecilidae are familiar aquarium fishes, e.g. guppies, mollies, platys and swordtails, species within Heterandria are not commonly kept as pets. Somewhat more frequently found in aquaria is H. formosa, perhaps because it is one of the smallest known fish species in the world; its diminutive males are probably the smallest fish easy to keep as a pet. Species in this genus resemble egg-laying Cyprinodontoidei such as Fundulidae (topminnows) at first glance, and are thus sometimes called "killifish" though this is technically erroneous.