Environmental Niche Differentiation and Past Colonization Dynamics of Two European Freshwater Mussels (Unio crassus and Unio nanus)

Understanding species distribution and environmental niches is crucial for conserving endangered taxa. The recent taxonomic split of the European freshwater mussels U. crassus and U. nanus into distinct species requires a reassessment of their distinct ecologies for conservation. This study uses species distribution models (SDMs) to define and compare the environmental niches and reconstruct the distributions across six past time periods, starting from the mid-Pliocene Warm Period (mPWP, ca. 3.205 Ma) to the present. Our results reveal significant environmental niche differentiation between the two species, with U. crassus occupying a broader environmental niche primarily influenced by annual mean temperature and precipitation in the warmest quarter. In contrast, U. nanus shows a narrower niche shaped by temperature seasonality, mean diurnal range, annual mean temperature, and precipitation seasonality. Paleodistribution models indicate that during the Last Glacial Maximum (LGM, ca. 21 ka), U. crassus persisted in multiple southern refugia, whereas U. nanus was restricted to a single western refugium. These contrasting glacial histories led to divergent post-glacial colonization routes, explaining their current genetic patterns and partially overlapping ranges. By identifying present environmental hotspots, this research provides an essential framework for developing targeted, species-specific conservation strategies for these freshwater mussels.