Overview sketch of the Cueva del Azufre system. Poecilia mexicana populations sampled for the present study originated from non-sulfidic surface sites (white circles), Arroyo Bonita (AB), Arroyo Tres (AT), Río Oxolotan (RO) and Río Amatan (RA), sulfidic surface sites (gray circles), El Azufre I and II (EA I, EA II) and the sulfidic Cueva del Azufre (CdA; black circle). In Tapijulapa, Río Amatan joins Río Oxolotan to form the Río Tacotalpa. Below: overview of the Cueva del Azufre with different cave chambers sampled for this study (V, X, and XIII). Insert: surface (above) and cave dwelling P. mexicana female.