Invaders versus endemics: alien gastropod species in ancient Lake Ohrid

Species invasions into ancient lakes are an important but little understood phenomenon. At ancient Lake Ohrid, a systematic assessment of invasive mollusc species using morphological and genetic data was conducted from 2003 to 2012. Two globally invasive gastropod species, Physa acuta and Ferrissia fragilis, have recently been discovered at 4 out of 386 sites. These sites are anthropogenically impacted. The invasive species co-occur with endemics. Phylogenetic analyses of populations from native and invaded ranges of both species confirmed their identities and provided insights into their invasion histories. Accordingly, P. acuta is genetically more diverse than F. fragilis. Both species are currently present in a considerable number of lakes on the Balkan Peninsula. Possible future trends in Lake Ohrid and the Balkans are discussed and further spread of both species is likely. Given the ongoing environmental change in Lake Ohrid, the number of observations of non-indigenous or other widespread species will probably rise in the coming years and such species and their impact on native species should be carefully monitored. Moreover, ancient lakes with recurrent invasions of alien species might serve as interesting model systems for the study of important topics of invasion biology.