How an endemic became widespread: the peculiar situation of the groundwater amphipod Niphargus boulangei Wichers, 1964 (Amphipoda, Niphargidae)

Niphargus boulangei (Amphipoda, Niphargidae), a rare groundwater amphipod from the northern continental range of the genus, was long known only from Pas-de-Calais (French Channel coast). Recent phylogenetic studies added records from Moselle (France) and Wallonia (Belgium). We molecularly screened available but undetermined Niphargus of North-Western Europe and collated all available information on the ecology, genetic diversity and distribution of the species. By adding further genetically validated specimens, a total of 11 populations with overall three directly connected COI haplotypes can be recognized from two highly disjunct distribution areas: Pas-de-Calais (Channel coast, France; four sites with one haplotype) and the Greater Region (Wallonia in Belgium, Luxembourg, Moselle in France; seven sites and all three COI haplotypes). Most records within the Greater Region are situated within the catchment of the Meuse River. The most likely evolutionary scenario to explain the observed disjunct distribution pattern considers the Fleuve Manche paleo-river system to have acted as a passive vector for the transportation of sediments of continental origin, which were flushed downstream during glacial periods until they reached the Weald-Artois anticline – a massive geomorphological barrier to fluvial sediments. The Meuse was a tributary of this paleo-catchment, connecting the Greater Region (potential source populations) with Pas-de-Calais (potential founding populations). However, the generally low genetic diversity of the species points to historical population bottleneck events or suggests that core populations might still be undiscovered. Our findings also support the hypothesis that narrow-range endemics may not exist for groundwater amphipods in north-western Europe.