Jenynsia is a genus of livebearing fish closely related to the four-eyed fish Anableps, classified in the family Anablepidae or, in some treatments, its own family Jenynsiidae, native to the southern half of South America, with atlas records from Brazil, Argentina, Uruguay, and Bolivia. The 16 species in the atlas are small, running roughly 1.5–3 in, and are found in a range of fresh and brackish waters, from coastal lagoons to inland streams and ponds. Like their Anableps relatives, Jenynsia species are livebearers with internal fertilization via a modified male anal fin (a gonopodium), and the group shows a notable asymmetry in that gonopodium — it is fixed to one side in a given male, a quirk tied to the fish's mating mechanics.
Jenynsia is a genus of modest aquarium presence compared to its livebearer relatives Poecilia and Xiphophorus, kept mainly by hobbyists with an interest in South American or brackish-tolerant livebearers rather than being a mainstream trade staple; species such as J. lineata are the ones most likely to be encountered. The conservation picture within the genus is notably uneven: alongside a majority of Least Concern species, the atlas tally includes five species rated Endangered and one Critically Endangered, a heavier threat burden than most livebearer genera, reflecting the vulnerability of small-range species tied to specific coastal and inland South American waters.