Genetic Diversity and Differentiation of Irish Freshwater Pearl Mussels (Margaritifera margaritifera): Identification of Conservation Units and Recommendations for the Management of Wild and Hatchery Populations

The freshwater pearl mussel Margaritifera margaritifera (FPM) has substantially declined throughout Europe. It is a target species in conservation of freshwater biodiversity, with several restoration and captive breeding efforts in place. Understanding the spatiotemporal patterns of genetic diversity in relation to environmental factors is crucial in increasing the success of such projects. Based on nonlethal haemolymph sampling from 42 (sub)populations throughout Ireland, a comprehensive assessment of the genetic diversity and differentiation of Irish FPM was realized. Given the previous identification of a genetic FPM diversity hotspot in Ireland, the debate about (sub)species ranks related to the co-occurrence of M. margaritifera and margaritifera durrovensis and the establishment of captive breeding efforts, the island setting of Ireland provided an ideal study case. Genetic differentiation of Irish freshwater pearl mussel populations suggests the existence of four distinct genetic clusters, which should be managed as separate conservation units. In a few highly inbred populations with occurrence of multiple individuals with the exact same genotypes, genetic drift likely explains the strong differentiation from other populations. Genetic diversity was particularly high in the most intact rivers at the west coast with occurrence of salmon as a host fish. In contrast, genetic variability was much lower in highly degraded rivers in the south-eastern part of Ireland where current census population sizes are low. The identified clusters and their different levels of diversity require conservation-unit specific prioritization: Habitat and host fish conservation are key for the genetically most diverse Western Irish FPM populations to avoid genetic drift effects and population bottlenecks, whereas the unique but less diverse FPM requires special attention to avoid further erosion of their variability. Captive breeding should maintain the high level of genetic variation and may also include representation of additionally identified conservation units in the future.