Population genomics of freshwater gastropods in the family Pleuroceridae

Population genetics research is often a foundation of effective conservation efforts. Yet, such research is often missing for critically imperiled freshwater gastropods. Recently, several population genomic studies revealed that populations of Pleuroceridae species often harbor high amounts of genomic diversity, even when the species has undergone severe range contraction. To test for this pattern in additional species, we generated genomic data for Elimia crenatella and Elimia capillaris. Both species have undergone greater than 95% range reduction and only one population of each species remains. We found that both species harbor relatively high amounts of genetic diversity in their remaining populations. Furthermore, genomic data do not indicate that remaining populations have undergone severe bottlenecks as might be expected given such drastic species-wide range contraction. By comparing these data to past studies, we developed a broad hypothesis to explain how pleurocerid species respond to habitat degradation: The Rapid Localized Extirpation Hypothesis. This hypothesis postulates that pleurocerid populations will persist without showing evidence of decline until a critical threshold in habitat quality is met; crossing the threshold then results in rapid and complete extirpation. Future work should test and possible expand on this hypothesis, which will likely include examining habitat requirements for pleurocerids, as basic toxicology and critical thermal limits are unknown for most species. Conservation efforts for pleurocerids should prepare for rapid extirpation of populations if habitat degradation occurs. However, high genetic diversity of remaining populations offers promise for management actions like captive propagation and reintroduction.