The Impact of Glacial Disturbance History Upon the Genetic Diversity of <i>Unio crassus</i> and <i>Unio nanus</i> in Europe and Implications for Conservation

Historically, the thick-shelled river mussel (<i>Unio crassus</i> agg. complex) was considered a single, widespread species across Europe. However, recent phylogenetic taxonomic revisions have delineated 12 species from this complex, including <i>Unio crassus</i> (s. str. Philipsson in Retzius, 1788) and <i>Unio nanus</i> (Lamarck, 1819 stat. rev.), which exhibit substantial range overlap and broad European distributions. Understanding their fine-scale genetic diversity, population structure, and potential for recent or ancient hybridization is critical for effective conservation planning. This study investigated the genetic diversity and structure of <i>U. crassus</i> and <i>U. nanus</i> across Europe, examining the influence of glacial disturbance history and host-fish associations. Using mitochondrial (COI) and nuclear (microsatellite) markers on 60 populations, we revealed a discordance between mitochondrial and nuclear structuring, suggesting ancient introgression. Crucially, no evidence of recent hybridization was detected between <i>U. crassus</i> and <i>U. nanus</i>. We found significantly higher nuclear genetic diversity in <i>U. crassus</i> compared to <i>U. nanus</i>. Our findings indicate an older Black Sea-Caspian Sea divergence and ancient introgression between <i>U. nanus</i> and <i>U. crassus</i>, as well as distinct postglacial colonization routes: a Western route for <i>U. nanus</i> and an Eastern route for <i>U. crassus</i>, converging in a secondary contact zone. Our results highlight the strong influence of host-fish associations and glacial history in shaping the genetic patterns of these mussels, underscoring the need to incorporate intraspecific genetic diversity into conservation strategies. As shell morphology proved unreliable for species identification, we recommend DNA barcoding for reliable species recognition and suggest further research into host-fish preferences to improve conservation efforts.