• adaptation;
  • competitive exclusion;
  • conservation biology;
  • extremophiles;
  • fish communities;
  • hydrogen sulphide

In the Baños 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 H2S. It is suggested that the fishes endemic to the Baños del Azufre are highly specialized, and competitive exclusion allows them to survive only in this particular habitat type.