Evolution in Extreme Environments: Replicated Phenotypic Differentiation in Livebearing Fish Inhabiting Sulfidic Springs

Abstract

We investigated replicated ecological speciation in the livebearing fish Poecilia mexicana and P. sulphuraria (Poeciliidae), which inhabit freshwater habitats and have also colonized multiple sulfidic springs in southern Mexico. These springs exhibit extreme hypoxia and high concentrations of hydrogen sulfide, which is lethal to most metazoans. We used phylogenetic analyses to test whether springs were independently colonized, performed phenotypic assessments of body and gill morphology variation to identify convergent patterns of trait differentiation, and conducted an eco-toxicological experiment to detect differences in sulfide tolerances among ecotypes. Our results indicate that sulfidic springs were colonized by three different lineages, two within P. mexicana and one representing P. sulphuraria. Colonization occurred earlier in P. sulphuraria, whereas invasion of sulfidic springs in P. mexicana was more recent, such that each population is more closely related to neighboring populations from adjacent nonsulfidic habitats. Sulfide spring fish also show divergence from nonsulfidic phenotypes and a phenotypic convergence toward larger heads, larger gills, and increased tolerance to H 2 S. Together with previous studies that indicated significant reproductive isolation between fish from sulfidic and nonsulfidic habitats, this study provides evidence for repeated ecological speciation in the independent sulfide spring populations of P. mexicana and P. sulphuraria. K E Y W O R D S :Ecological speciation, hydrogen sulfide, hypoxia, local adaptation, morphological differentiation, mtDNA phylogeny, natural selection, Poecilia.

Patterns of Macroinvertebrate and Fish Diversity in Freshwater Sulphide Springs

“…Similar to variation in H 2 S concentrations, springs substantially vary in temperature, with reported values ranging from around 5 °C to over 100 °C [35,40,41,47,48,57,76,81]. Temperature variation appears to be related in part to the geographic location of springs as well as the ultimate sources of H 2 S production.…”

“…Conversely, peak concentrations of H 2 S have been reported to exceed 7000 μM in some springs [79]. Currently available data suggest that H 2 S concentrations in most springs lie in between these extremes, with a large number of springs having concentrations towards the lower end of the spectrum (i.e., <2500 μM H 2 S [40,41,44,47,76,78,80]). …”

“…Even though not described as distinct species, several populations particularly in the family Poeciliidae also represent phenotypically divergent and genetically distinct ecotypes, indicating that they represent locally adapted populations restricted to H 2 S-rich environments [67,70,76]. However, additional research is required to test for the presence of local adaptation in the Cyprinodontidae and Synbranchidae, as at least some populations may be temporary or represent sinks that solely persist in H 2 S-rich environments because of continuous immigration from adjacent non-sulphidic habitats.…”

“…During this process, groundwater is being enriched in H 2 S and ultimately discharged through surface springs [40,43]. Table 1 and [35,40,41,[44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] [76], but even such low concentrations are biologically relevant and considered toxic for most metazoans [16]. Conversely, peak concentrations of H 2 S have been reported to exceed 7000 μM in some springs [79].…”

“…Temperature variation appears to be related in part to the geographic location of springs as well as the ultimate sources of H 2 S production. Sulphide springs are often also characterised by increased concentrations of bicarbonate, calcium sulphate, sodium chloride, and other ions (leading to substantial increases of specific conductivity [39]), and by lower pH likely caused by the presence of sulphuric acid from chemical and bacterial H 2 S oxidation (although pH reductions are dependent on the buffering capacity of the water in the region) [57,76,82]. Finally, upon the discharge of sulphidic water at the surface, H 2 S spontaneously oxidizes in water, causing and aggravating hypoxic conditions in aquatic systems [83,84].…”