Resilient by nature: managed rearing does not erode physiochemical tolerances of an extremophile fish

Populations in managed care are valuable resources that complement in situ conservation efforts, but adaptation to captive conditions and other domestication effects present concerns for conservation. Many populations of fish adapted to sulfide springs are highly endemic and imperiled, potentially benefiting from ex situ conservation efforts. However, it is challenging to maintain natural conditions in the laboratory as hydrogen sulfide (H2S) is highly toxic and coincides with severe hypoxia. Here, we tested whether long-term standardized rearing of sulfide spring fishes leads to the loss of H2S and hypoxia tolerances. We compared the tolerances of Poecilia mexicana (Poeciliidae) from sulfidic and non-sulfidic habitats that were reared in the laboratory for 18 years and wild-caught fish from the same sites. Both H2S and hypoxia tolerances were maintained in laboratory-reared fish from the sulfidic habitat. Additionally, fish from a non-sulfidic cave site, evolutionarily derived from a sulfidic population, exhibited higher H2S and hypoxia tolerances than fish from the non-sulfidic surface site. While domestication can lead to the loss of adaptations to extreme environmental conditions, our research indicates this is not a concern in laboratory stocks of P. mexicana, which retained tolerance of H2S and hypoxia despite not experiencing these stressors for approximately 40 generations.