Population genetics and geometric morphometrics of freshwater snail Segmentina nitida reveal cryptic sympatric species of conservation value in Europe

Segmentina nitida is a rare European freshwater snail of drainage ditches and marshland, which has seen a marked decrease in range (~80%) over the last 100 years in the UK. This has been attributed to over-dredging of drainage ditches for land management, conversion of grazing marshes to arable farmland, as well as eutrophication. Segmentina nitida is identified as a priority species in the UK Biodiversity Action Plan (UKBAP) that recommends further research to inform reintroduction and translocation for its conservation. We used microsatellite, nuclear (ITS2) and mitochondrial (COI) markers to investigate population structure in S. nitida individuals sampled from Poland, Germany, Sweden, and the UK to identify differences within and between populations. Landmark-based geometric morphometrics of S. nitida shells was used to determine if phenotypic variation followed genetic differentiation. Two distinct genetic lineages of S. nitida were identified in ITS and COI phylogenies and in cluster analysis of microsatellite markers, one present in eastern Europe (Poland, Sweden), and one in western Europe (UK, Germany), with lineages co-occurring in German populations. No genetic admixture was observed in German populations containing both lineages. These two lineages were also distinct in shape, with ~85% of shells assigned to the correct lineage in a discriminant analysis of Procrustes shape coordinates. We infer that S. nitida includes at least one sympatric cryptic species. We discuss the implications of these findings on the conservation status of S. nitida in the UK and Europe.