Scientists have characterised the habitat of two rare and endangered poeciliid fishes from Mexico that are known only from sulphidic (with high levels of hydrogen sulphide) habitats.
Michael Tobler, Rdiger Riesch, Francisco Garca de Len, Ingo Schlupp and Martin Plath provide data on the water chemistry of a number of sulphidic springs about 10 km west of Teapa (Tabasco, Mexico).
These springs enter a creek forming an area that is locally known as the Baos del Azufre and are home to two endemic, critically endangered species of poeciliids: the sulphur molly Poecilia sulphuraria and the largemouth mosquitofish Gambusia eurystoma.
Hydrogen sulphideThe authors found the hydrogen sulphide concentrations at the Baos del Azufre comparable to, and in places even higher than, those of the nearby Cueva del Azufre system and habitats of deep-sea hydrothermal vents inhabited by metazoans.
The two poeciliid species were the only fish found in microhabitats that had hydrogen sulphide.
Of the two species, Poecilia sulphuraria is the only one found in the sulphurous areas directly below larger hydrogen sulphide springs.
The means by which the fish is able to tolerate high hydrogen sulphide levels is unknown, although the short-term survival of fishes in water containing hydrogen sulphide has been shown to depend on the possibility to breath air or perform aquatic surface respiration and P. sulphuraria has been observed to perform aquatic surface respiration in areas of high hydrogen sulphide concentration.
Lip appendagesThis species possesses two conspicuous lip appendages, a character that is not known from any other poeciliid.
Although previously hypothesized to function as taste organs, a more plausible explanation is that the appendages are an adaptation to the extremely hypoxic environment and maximize the efficiency of aquatic surface respiration.
The extremely wide mouth of G. eurystoma may serve a similar function as the lip appendages in P. sulphuraria.
The authors write Poecilia sulphuraria and G. eurystoma certainly belong to the few truly extremophile vertebrates and are a prime example of adaptive evolution in an extreme habitat.
Gambusia eurystoma is closely related to Gambusia sexradiata, which is common in the region...Poecilia sulphuraria is a sister species of P. mexicana, which was abundant in the non-sulphurous waters around the Baos del Azufre in the present collections.
Although both species could be caught only a few metres apart, they were strictly separated in terms of water chemistry, suggesting niche partitioning between the congeners.
Migration of P. mexicana into sulphidic habitats is probably limited by the presence of . Likewise, although P. sulphuraria can survive in non-sulphidic water, competitive exclusion possibly prevents this species from expanding its range.
For more information, see the paper: Tobler, M, R Riesch, FJ Garca de Len, I Schlupp and M Plath (2008) Two endemic and endangered fishes, Poecilia sulphuraria (Alvarez, 1948) and Gambusia eurystoma Miller, 1975 (Poeciliidae, Teleostei) as only survivors in a small sulphidic habitat. Journal of Fish Biology 72, pp. 523"533.