Least Killifish

Heterandria formosa

The least killifish (Heterandria formosa) is one of the smallest vertebrates in the world, belonging to the family Poeciliidae. Males reach only about 2 cm and females about 3 cm. Despite its name, it is not a true killifish but a livebearer related to guppies and mollies. They have an olive-brown body with a distinctive dark horizontal stripe and a dark spot on the dorsal fin. This species inhabits densely vegetated shallow freshwater and brackish habitats at depths of 0-1 meter across the southeastern United States, from North Carolina through Florida to Louisiana. Despite their tiny size, they are voracious micro-predators feeding on small invertebrates, algae, and detritus. Females give birth to live young through a unique process of superfetation, carrying embryos at multiple developmental stages simultaneously. Their remarkable miniature size makes them a fascinating find for observant freshwater divers.