DREISSENA POLYMORPHA: A BLACK SEA NATIVE, BUT HOW RECENT A BALTIC DENIZEN?

The zebra mussel, Dreissena polymorpha, one of the most widely known invaders in freshwater aquatic ecosystems, was unintentionally introduced to northwest Russia, central and western Europe, Scandinavia, Britain, Ireland, and North America. During the early Holocene, the range of D. polymorpha was limited to the Black, Azov, Caspian, and Aral Seas, as well as estuaries and lower and middle reaches of the Ponto-Aralo-Caspian rivers. However, during the last interglacial, its range limit was situated farther northward. It is believed that D. polymorpha penetrated into the Baltic Sea basin (Curonian and Vistula Lagoons) only in the early 1800s via human-mediated invasion corridors: the Oginskiy Canal connecting Nemunas and Pripet Rivers (opened in 1769) the Bug-Pripet Canal (1775). However, recent geological investigations along the Curonian and Vistula Spits, as well as the Curonian Lagoon, uncovered fossil shells of D. polymorpha that yielded radiocarbon ages varying from approximately 300 to 2,100 radiocarbon years before present (BP). These new findings contradict the widely held opinion about the relatively recent (~200 years) appearance of zebra mussel in the Baltic. We propose two hypotheses to explain the new radiocarbon dates of D. polymorpha. The first considers that this species spread naturally into the Baltic basin during Late Holocene as the rivers draining into the Baltic Sea captured the tributaries of those in the Black Sea basin. The gap in the recently obtained ages corresponds to the Little Ice Age and may be explained by unfavorable climatic conditions for this species. The second hypothesis involves the anomalously large reservoir effect. Bulk radiocarbon ages of several present-day molluscs from the Curonian Lagoon range from 1,400 to 2,100 BP. This may be due to the fact that the water of the Curonian Lagoon is enriched in old carbonates much more than in the Baltic Sea, where basin effect is estimated as 400 years. D. polymorpha from the outcrops along the Vistula Spit yielded an AMS date of 1,340 BP, suggesting a similar reservoir effect for the two lagoons. Regardless of which scenario proves to be correct, our findings have important implications for the recent biogeography and paleoecology of the Ponto-Caspian and Baltic basins.