Samples of Austrolebias nigrofasciatus ( n = 103), an endangered species of annual fish endemic to a small area of the Patos‐Mirim lagoon system encompassing the São Gonçalo Channel lowlands, were collected from eight isolated temporary ponds, four located at the known distribution range of the species and four located along the Piratini River lowlands, where morphologically different individuals were found. In the laboratory,...