Like Anableps species, they are onesided livebearers: some sources indicate that they only mate on one side, right-"handed" males with left-"handed" females and vice versa.

[3] Species of the genus are distributed in the Río de la Plata Basin and Atlantic coastal drainages from Río Negro Province, Argentina, to the city of Rio de Janeiro, Brazil, and in the Andean drainages of northwest Argentina and southern Bolivia.

Members of the subgenus Plesiojenynsia Ghedotti, 1998, are distributed in the uplands of southern Brazil.

[1] Jenynsia species are diagnosable by the possession of an unscaled tubular gonopodium formed chiefly by the third, sixth, and seventh anal-fin rays and by the possession of tricuspid teeth in the outer mandibular series in adults.

[4] The maximum length in these species is up to 12 centimetres (5 in) in females and about 4 cm (2 in) in males.