Occurs mostly in brackish water lagoons in sand-spit areas; Phalloptychus januarius and Poecilia vivipara were found in the same habitat .
Dorsal soft rays (total): 8 - 10; Anal soft rays: 10; Vertebrae: 29 - 31. This speices is distinguished from its congeners by the following characters: except for J. lineata, J. onca, J. sanctaecatarinae, J. maculata, J. luxata, by its colour pattern of round or dash-shaped marks forming multiple irregular rows; differs from J. onca and J. sanctaecatarinae by having a swelling between the urogenital opening and the base of the anal fin of females (vs. absent); differs from J. onca by the not having a distinct dorsal convex expansion at subdistal segments of right hemitrichs of sixth anal-fin ray of adult males (vs. present); differs from J. maculata by having the left and right hemitrichs of anal-fin ray six laterally identical in adult males (vs. not laterally identical); differs from J. luxata by having the medial processes of left and right pelvic bones overlapping at the ventral midline (vs. reduced processes not overlapping); and from J. lineata by the dorsal postcleithrum being three times deeper than wide (vs. less than twice as deep as wide) and with half of the caudal peduncle of females having rows of chromatophores segmented into unaligned spots (vs. spots forming lines) .Cross section: oval.
Behavioural data for Jenynsia Darwini is currently being reviewed.
Diet information for Jenynsia Darwini is currently being compiled.
Reproduction data for Jenynsia Darwini is currently being compiled.
Freshwater; brackish; benthopelagic. Tropical South America: Brazil.
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