The Least Killifish, or Dwarf Livebearer, (Heterandria formosa) is a species of livebearing fish within the family Poeciliidae. This is the same family that includes familiar aquarium fishes such as guppies and swordtails. The Least Killifish is not as commonly kept in aquaria as these species. The Least Killifish is one of the smallest fish in the world (7th smallest as of 1991), and is the smallest fish found in North America.
The Least Killifish is the only member of the genus Heterandria to be found in the United States. Its range covers southeastern United States, from South Carolina south to Georgia and Florida, and through the Florida Gulf Coast to Louisiana. It is one of the few aquarium fishes to come from North America.
The Least Killifish is one of the smallest fish and smallest vertebrates known to science. Males grow to about 2 centimeters (0.8 inches), while females grow a little larger, to about 3 centimeters (1.2 inches).
The fish is generally an olive color, with a dark horizontal stripe through the center of the body. There is also a dark spot on the dorsal fin and females also have a dark spot on their anal fin. Like most poeciliids, males' anal fins are modified into a gonopodium that is used for impregnating females during mating.
Like most poeciliids, the Least Killifish 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 young are released free swimming. Least Killifish have a unique breeding strategy even among livebearers. Rather than all 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.