Poecilia vivipara, sometimes called the southern molly, is a small euryhaline livebearer fish distributed along the Atlantic coast of South America.

[6] Individuals from habitats with higher water salinity tend to grow faster and larger,[7][8] and these differences in growth patterns are partly heritable.

It is traditionally considered to have been introduced to Brazil's Fernando de Noronha archipelago to control mosquito larvae during the construction of the Second World War military bases but the species's ability to survive in saltwater makes it possible that the fish colonized the area naturally.

[11] It tolerates varying degrees of salinity, ranging from freshwater to hypersaline,[9] but is most common in slightly brackish and rare in fresh water.

[2] Freshwater habitats tend to feature abundant marginal vegetation while aquatic plants are typically absent from saltwater sites.

Heavily infected fish are lethargic and more easily fall prey to the common snook, Centropomus undecimalis, which is the parasite's definitive host.

Possible predators in brackish waters include cichlids Geophagus brasiliensis and Australoheros facetus, which have been observed preying on young but not adult P. vivipara in laboratory settings.

[11] Unlike the common guppy, P. vivipara males rely on opportunistic (sneak and chase) matings rather than courtship displays.