Habitat preferences and genetic diversity of the amphipod Gammarus roeselii across the Eastern Alps and western Pannonian Basin

Freshwater amphipods often exhibit cryptic diversity and are undergoing range shifts driven by environmental changes. Gammarus roeselii, a species inhabiting streams, rivers and lakes across Europe, diversified into ancient genetic lineages in the Balkan Peninsula and the Pannonian Basin, with only one lineage colonizing Central and Western Europe after the ice ages. We investigated the distribution and genetic diversity of G. roeselii by sampling over 1,000 sites across the eastern Alpine region and the Western Pannonian Basin. Cytochrome oxidase I barcoding assigned all sequenced G. roeselii (528 individuals from 174 sites) to the Central-Western European lineage. The occurrence of G. roeselii was associated with low elevation, high summer temperature and gentle stream and river slopes and was biased towards downstream reaches and rivers with large drainage sizes. Its distribution partially overlapped with G. fossarum, which predominates in cooler, faster-flowing streams. Species distribution modelling under future climate scenarios predicted a range expansion of G. roeselii into current G. fossarum habitat. Genetic diversity patterns are consistent with longstanding stable populations in the Southwestern Pannonian Basin, a post-glacial range expansion across alpine forelands, and recent colonization of some alpine valleys.