Establishing conservation units to promote recovery of two threatened freshwater mussel species (Bivalvia: Unionida: Potamilus)

Population genomics has significantly increased our ability to make inferences about microevolutionary processes and demographic histories, which have the potential to improve protection and recovery of imperiled species. Freshwater mussels (Bivalvia: Unionida) represent one of the most imperiled groups of organisms globally. Despite systemic decline of mussel abundance and diversity, studies evaluating spatiotemporal changes in distribution, demographic histories, and ecological factors that threaten long-term persistence of imperiled species remain lacking. In this study, we use genotype-by-sequencing (GBS) and mitochondrial sequence data (mtDNA) to define conservation units (CUs) for two highly imperiled freshwater mussel species, Potamilus amphichaenus and Potamilus streckersoni. We then synthesize our molecular findings with details from field collections spanning from 1901 to 2019 to further elucidate distributional trends, contemporary status, and other factors that may be contributing to population declines for our focal species. We collected GBS and mtDNA data for individuals of P. amphichaenus and P. streckersoni from freshwater mussel collections in the Brazos, Neches, Sabine, and Trinity drainages ranging from 2012 to 2019. Molecular analyses resolved disputing number of genetic clusters within P. amphichaenus and P. streckersoni; however, we find defensible support for four CUs, each corresponding to an independent river basin. Evaluations of historical and recent occurrence data illuminated a generally increasing trend of occurrence in each of the four CUs, which were correlated with recent increases in sampling effort. Taken together, these findings suggest that P. amphichaenus and P. streckersoni are likely rare throughout their respective ranges. Because of this, the establishment of CUs will facilitate evidence-based recovery planning and ensure potential captive propagation and translocation efforts are beneficial. Our synthesis represents a case study for conservation genomic assessments in freshwater mussels and provides a model for future studies aimed at recovery planning for these highly imperiled organisms.

Cumulative Spring Discharge and Survey Effort Influence Occupancy and Detection of a Threatened Freshwater Mussel, the Suwannee Moccasinshell

Freshwater mussels (Unionidae) are among the most imperiled groups of organisms in the world, and the lack of information regarding species distributions, life-history characteristics, and ecological and biological requirements may limit the protection of remaining mussel populations. We examined the influence of hydrologic factors on the occurrence of the Suwannee Moccasinshell Medionidus walkeri, a federally threatened freshwater mussel species, endemic to the Suwannee River Basin in Georgia and Florida. We also evaluated the influence of survey effort on detection of Suwannee Moccasinshell during field surveys. We compiled all recent (2013–2016) mussel survey records in the Suwannee River Basin. We calculated cumulative discharge contributed by upstream springs for each of 220 survey locations. We combined the spring discharge predictor variable with Suwannee Moccasinshell detection and nondetection data from each survey location to develop a suite of occupancy models. Modeling results indicated that detection of Suwannee Moccasinshell during surveys was strongly and positively related to survey effort. Modeling results also indicated that sites with cumulative spring discharge inputs exceeding ∼28 cubic meters per second were most likely (i.e., predicted occupancy probabilities >0.5) to support Suwannee Moccasinshell populations. However, occupancy declined in the lowermost reaches of the Suwannee mainstem despite high spring discharge inputs, presumably due to greater tidal influences and differences in physicochemical habitat conditions. Historical localities where Suwannee Moccasinshell has presumably been extirpated are all devoid of springs in their upstream watersheds. We hypothesize that springs may buffer extremely tannic, and at times polluted, surface waters, in addition to maintaining adequate flows during periods of drought, thereby promoting the persistence of Suwannee Moccasinshell populations. Our study suggests that springs are a critical resource for Suwannee Moccasinshell and may be more important for conservation planning than was previously recognized.