The southeastern United States is home to over 500 freshwater fishes, many of which only survive in pristine waters. Small non-migratory freshwater fishes and invertebrates are excellent bioindicators because they respond to local ecological disturbances that are relevant to public water supply (e.g., hydrofracking, bacterial overgrowth, stormwater runoff, etc.). The Pygmy Sunfishes represent a unique component of the southeastern aquatic biodiversity hotspot, and more than half of the known species are sensitive to such disturbance. Dr. Michael W. Sandel, postdoctoral trainee in the department of biostatistics, section on statistical genetics, at the University of Alabama at Birmingham, recently presented the first phylogeny of seven Pygmy Sunfish species, which begins to answer the question of how so many species have come to occupy the same area.
Dr. Sandel and his team conducted a phylogenetic analysis, which reveals the isolated and federally protected Spring Pygmy Sunfish to be the most evolutionarily distinct member of the genus. All other species are primarily restricted to rivers of the Coastal Plain, a land area previously covered by the Atlantic Ocean. Dr. Sandel and co-authors noticed that male Pygmy Sunfishes living within the range of related species tended to be very colorful during courtship, while those living outside the range of related species were relatively dull. They tested the hypothesis that shape differences—which are more easily quantified—would show a similar pattern of sexual dimorphism.
A new index of interspecific sexual shape dimorphism revealed that co-habitating species share fewer sexually dimorphic traits, on average, than species that live outside the range of congeners. When the data was plotted on a time-calibrated phylogeny, it was evident that sexually dimorphic traits were evolving faster among species that swim in the same waters. From an ecological perspective, this result suggests that small, freshwater fishes have evolved sexual dimorphic traits to quickly differentiate potential mates (same species) from competitors (different species). The researchers suggest that the unique geological history of the southeast region has promoted the formation of ecologically diverse species and the evolution of sexually dimorphic traits helps to maintain this diversity after related species colonize the same drainage systems. With a better understanding of the factors that generate aquatic biodiversity, resource managers are better able to prevent extinction of bioindicator species, which simultaneously protects public water supply. “Interspecific Relationships and the Evolution of Sexual Dimorphism in Pygmy Sunfishes (Centrarchidae: Elassoma)” was published online in April in Molecular Phylogenetics and Evolution.