Pigment cell retention in cavernicolous populations of a Poecilia mexicana (Poeciliidae)

The Atlantic molly (Poecilia mexicana) is a small livebearing fish that has colonized two cave systems in the southern state of Tabasco, Mexico. Unlike many obligate cave-dwellers (i.e. troglobites) all cave P. mexicana retain some pigmentation, as well as a functional visual system. In the Cueva del Azufre the fish occupy habitats (i.e. cave chambers) that differ along a gradient of hydrogen sulfide (H₂S) concentration, as well as a patchwork of light exposure due to several large skylights. While the relationship between eye size and opsin expression with cave distance has been explored, the extent of differences in pigmentation has not yet been quantitatively evaluated. In this study we compared pigment cell (melanophore and xanthophore) count in wild-caught fish from one surface population (Arroyo Bonita) and two cave populations: chambers V (featuring a skylight) and chamber X (exists in perpetual darkness) of the Cueva del Azufre over a 120 day period. Surface populations had significantly higher total numbers of pigment cells than both cave populations, which did not differ significantly from each other. We speculate that skylights in the Cueva del Azufre, paired with a recent evolutionary origin of the cave population and genetic homogenization, have allowed for trait maintenance in cavernicolous P. mexicana.