Genetic structure of Irish freshwater pearl mussels (Margaritifera margaritifera and Margaritifera durrovensis): Validity of subspecies, roles of host fish, and conservation implications

Abstract

European freshwater pearl mussels are among the most critically threatened bivalve molluscs. Margaritifera margaritifera and Margaritifera durrovensis are endangered, and both occur in Ireland and are currently listed separately in Annex II of the European Habitats Directive. This study had the objective of guiding the conservation of freshwater pearl mussels in Ireland based on a genetic characterization of the most important Irish populations of both species using microsatellite and mitochondrial DNA (mtDNA) analyses. Margaritifera durrovensis from the Nore was found to have the highest genetic divergence from all the populations studied; however, there was substantial relatedness between the genetic constitution of the Nore and of central–eastern M. margaritifera populations, placing the Nore pearl mussel within the M. margaritifera taxon (Habitats Directive species code 1029). Two main conservation units of pearl mussel were identified: a mostly salmon‐dependent western cluster and a trout‐dependent central–eastern cluster. The genetic diversity of western Irish freshwater pearl mussels, as expressed by allelic richness and observed heterozygosity, was greater by a factor of two than in central European and central–eastern Irish populations, suggesting a genetic diversity hot spot and low effects of genetic drift and selection. The trout‐dependent central and eastern populations had much lower genetic variability, but had the greatest differentiation and uniqueness. Conservation of freshwater pearl mussels in Ireland should recognize the existence of a minimum of two conservation units (western and central–eastern) that differ in their use of host fish and in geographic isolation. Low levels of genetic drift and inbreeding in western populations should be secured by sustaining optimal habitat conditions favourable for the recruitment of mussels and their migratory salmonid hosts. The small population sizes of central–eastern populations and problems with recruitment require urgent action, e.g. by captive breeding and augmentation, to prevent any further erosion of their genetic variability.

Population structure, genetic diversity, and colonization history of the eastern pondmussel, Sagittunio nasutus, in the Great Lakes drainage

“…Other inland lake populations sampled in this study represent even less genetic diversity and appear to have been subsequently colonized by small founding populations; however, populations in this study that have lower genetic diversity may be the result of human impact as well as signatures of historical founding effects. Geist et al () found that Irish populations of Margaritifera margaritifera that had experienced a deterioration of habitat quality had lower diversity indices, some by more than a factor of two, compared with other Irish populations with fewer human impacts.…”

Developing a standard approach for monitoring freshwater pearl mussel (Margaritifera margaritifera) populations in European rivers

“…Freshwater pearl mussels are now probably extinct in a number of countries, including Poland and Lithuania (Moorkens, ), and many of the remaining populations in central and southern Europe are greatly reduced, highly fragmented, and often ‘functionally extinct’ owing to the lack of recent recruitment and low genetic diversity (Geist, ; Lois, Ondina, Outeiro, Amaro, & San Miguel, ; Stoeckle et al, ). Recent surveys in more northern latitudes have confirmed that many large populations with high genetic diversity remain (Geist et al, ; Geist & Kuehn, ; Geist, Söderberg, Karlberg, & Kuehn, ). For example, Scotland, Ireland, Sweden, and Finland each support more than 100 populations of freshwater pearl mussels, including several estimated to contain more than 1 000 000 individuals (Cosgrove et al, ; Moorkens, ; Oulasvirta, Leinikki, & Syväranta, ; Söderberg, Henriksson, Karlberg, & Norrgran, ).…”

Downstream transport and seasonal variation in freshwater pearl mussel (Margaritifera margaritifera) eDNA concentration

“…Freshwater pearl mussels are subject to substantial conservation effort (Geist, 2010) and monitoring programs across Europe (Boon et al, 2019). The species has been studied for its genetic population structure across Europe (Geist & Kuehn, 2005;Geist, Söderberg, Karlberg, & Kuehn, 2010;Stoeckle et al, 2017) which in Northern Europe is linked to host preference (Geist et al, 2018;Karlsson, Larsen, & Hindar, 2014).…”