In the Banos del Azufre in Tabasco, Mexico, only two poeciliid species, Poecilia sulphuraria and
Gambusia eurystoma, were found in sulphidic habitats, whereas adjacent non-sulphidic habitats
were characterized by a vastly different fish community. Exposure of fishes to water from
a sulphidic spring showed that all species except for P. sulphuraria quickly lost motion control, probably because of the toxic properties of H2O It is suggested that the fishes endemic to the Banos del Azufre are highly specialized, and competitive exclusion allows them to survive only in this particular habitat type.