Least Killifish

Heterandria formosa is the only member of its genus found in the United States. Its geographical range spans the southeastern United States, from North Carolina south to Georgia and Florida, and westward across the Florida Gulf Coast to Louisiana. In recent years, this species has been collected in eastern Texas. It is recorded to occur along the western portion of the Sabine River basin, according to North American Native Fishes (NANFA). It has also been collected as far west as Humble, TX in small sand pit ponds after the 2017 floods associated with Hurricane Harvey. It is one of the few aquarium fishes that is endemic to North America. Heterandria formosa live primarily in vegetated, slow moving or standing freshwater habitats, but this species also occurs in brackish waters.

Like most poeciliids, H. formosa is a livebearer. The male uses his modified anal fin, or gonopodium, to deliver sperm to the female. The fertilized eggs grow within the female until they hatch, and the free-swimming young are released into the water. Heterandria formosa has an uncommon breeding strategy even among livebearers: rather than all of the young being released at once, as many as 40 fry are released over a 10- to 14-day period, but occasionally over a longer period.

The effect of inbreeding on reproductive behavior was studied in H. formosa in at least one published work. One generation of full-sib mating was found to decrease reproductive performance and likely reproductive success of male progeny. Other traits that displayed inbreeding depression were offspring viability and maturation time of both males and females.