Hasemania piatan is described from the upper rio de Contas drainage, Bahia, northeastern Brazil. It can be easily distinguished from its congeners by having 18 principal caudal-fin rays. The new species differs further from congeners by a combination of seven branched dorsal-fin rays, six branched pelvic-fin rays, anal-fin base not covered by scales, presence of only five infraorbitals, and presence of a humeral blotch. It also can be distinguished by having 10-13 branched anal-fin rays, 27-32 scales on longitudinal series, 10-12 circumpeduncular scales, and one to three maxillary teeth.
Coastal drainage; Chapada Diamantina; Freshwater fish
