Saving what’s left: conservation genomics of Plethobasus, a critically endangered genus of freshwater mussel

Abstract Freshwater mussels (Order Unionida) are diverse, yet highly imperiled, bivalves. In the United States, about 10% of approximately 300 native species are extinct and around 60% of the remaining species are imperiled. Conservation practitioners are engaged in recovery efforts, but without genetic oversight these activities can be counterproductive. The freshwater mussel genus Plethobasus consists of three federally endangered species. We quantified and compared genetic diversity of two species, P. cooperianus and P. cyphyus , and sought to genetically confirm a putative sample of the extremely rare species P. cicatricosus . We found a single panmictic P. cooperianus population distributed across the species’ extant range, but two distinct P. cyphyus populations. Genetic diversity was similar for both species and effective population sizes ( N e ) were higher than in other species of freshwater mussels, potentially resulting from unsampled populations and/or long generation times. While our results do not support the identification of the putative P. cicatricosus sample genetically, they also do not call into question the validity of P. cicatricosus as a species. We suggest that further sampling efforts are required to determine whether viable populations of this species are extant. Our study provides essential information for guiding conservation activities for this critically imperiled genus.