Townsend et al. (2003) defined an extreme environmental condition as one that requires, of any organism tolerating it, costly adaptations absent in most related species. Physiochemical stressors such as thermal extremes, low oxygen contents, sharp salinity gradients, extreme acidity or alkalinity, or high concentrations of toxic chemicals can directly influence the composition of ecosystems both in zoogeographical and local scale. The ability of organisms to tolerate or avoid physiochemical stress is critical for their success in an extreme environment. Many fish species successfully colonized extreme habitats such as the deep sea, the arctic region or the Lake Natron in Africa. Are there any extreme habitats in Central America and have cichlids adapted to live in them?
'Cichlasoma' salvini inhabits a cave in Tabasco
On a field trip in January 2006, 'Cichlasoma' salvini could be recorded from a cave in southern Tabasco (Cueva Bocca near the village of Puxcatan). Besides this, the poeciliid Heterandria bimaculata and shrimp (probably Palemonetes sp.) could be found, but no cave adapted catfish or poeciliids that were expected. It is not clear so far, if "C." salvini permanently inhabits the Cueva Bocca. The fish recorded so far do not show any apparent differences from "C." salvini from adjacent surface populations.
The El Azufre and 'Cichlasoma' cf. salvini
A habitat that can definitely be considered as extreme is the Cueva del Azufre and its outflow (El Azufre) in Tabasco, México. Within the cave, absence of light alone can theoretically be considered as extreme. Moreover, the creek flowing through the cave is fed by several springs containing high concentrations of hydrogen sufide (H2S), which is highly toxic for all animal life. Thus it can clearly be classified as an extreme environmental factor. Measurements of H2S within the cave and the El Azufre revealed high concentrations (up to 300 microM, Tobler et al., in prep.) that are generally lethal or at least chronically toxic for animals.
Nonetheless, the Cueva del Azufre was previously known as habitat of a unique population of the Atlantic molly (Poecilia mexicana, Poeciliidae): The Cave molly with reduced eyes and pigmentation (Gordon & Rosen, 1962). P. mexicana also lives in the El Azufre. Furthermore, Ophisternon aenigmaticum could rarely been reported from the cave. Thus this fish can cope with the toxic environment somehow (how exactly is not clear yet, but behavioral adaptation might play a role: Plath et al., in prep.).
Although this system was intensively studied the last 30 years, no other fish species were recorded in the El Azufre. In fall 2004, Plath, Schlupp & Tobler reported a cichlid similar to "C." salvini (see 'C.' cf. salvini) from the sulfurous creek. Compared to P. mexicana, the cichlid occurs only in very low number (three cichlids were caught compared to several thousand poeciliids). This form can also be found in adjacent habitats without sulfur. It remains to be studied how this cichlid manages to survive in this extreme environment.
Other sulfurous habitats exist in South México, but so far these are very poorly studies. I'd be very interested in further reports of cichlids and any other fish species living in extreme habitats of Central America.
|El Azufre spring, Tacotalpa, Grijalva, a high hydrogen sufide (H2S) content spring near the village of Tapijulapa, Tabasco, México where few individuals of Cichlasoma salvini could be recorded.Photo by Michi Tobler||The El Azufre is directly connected to the Cueva del Azufre, where a unique population of Poecilia mexicana can be found: The Cave molly. Since it lives in complete darkness and toxic water, it is considered as one of the few extremophile vertebrates. Photo by Michi Tobler|