LAUSR

Lake Malawi is famous for the rapid speciation of cichlid fishes, but the potential diversification of its invertebrate fauna is poorly studied. Using two mitochondrial DNA sequence markers (partial ND1 and Cyt b genes), we investigated the population genetic structure of the only known species of freshwater crab inhabiting Lake Malawi (Potamonautes lirrangensis (Rathbun, 1904)). We detected little overall genetic differentiation among different sampling sites (pairwise ΦST-values = 0.00–0.13). Genetic differentiation between sampling sites was better explained by linear distances than by shoreline distances, suggesting that ‘sweepstake dispersal’ between western and eastern shores occurs. Moreover, several population genetic parameters (Tajima’s D, Fu’s FS, Fay and Wu’s H and mismatch distribution analysis) suggest a recent population expansion, and Bayesian skyline plot analysis confirmed a sudden increase of the effective population size between 70 and 30 ka. Genetic diversity decreased towards the southern, shallower part of the lake, suggesting a more recent colonization of the southern shores. This finding is in line with hypotheses on Lake Malawi’s paleogeography suggesting that the lake largely desiccated during Pleistocene East African megadroughts and re-expanded southwards only recently after ~ 70 ka.