DNA barcoding sheds light on hidden subterranean boundary between Adriatic and Danubian drainage basins

In principle, the distribution of freshwater fauna follows the extent of drainage basins, but various historical and concurrent factors weaken this correspondence. Subterranean karstic drainages can be particularly baffling as their boundaries are decoupled from the surface relief and change with the level of the karstic groundwater. We explored how the distribution and genetic structure of cave fauna correlate to data obtained by conventional groundwater tracers in the Danubian-Adriatic divide in the classical Karst between Postojna and Trieste. Here, waters from the Pivka Polje drain to either basin, depending on the water level. This ambiguity is reflected by the distribution of several aquatic cave species that disperse from one drainage to the other. However, using DNA barcoding methods, we found a clear-cut separation between the closely related subterranean aquatic isopods Asellus aquaticus on the Danubian and Asellus kosswigi on the Adriatic side. The boundary between them agrees with the established hydrology at normal water levels. Further phylogeographic and demographic analyses suggest that the superficial concurrent correspondence has a deeper historical component. We inferred that the Adriatic species did not reach its present boundary by recent range dispersal, but rather has remained there as relict from Pleistocene times. Today, dispersal from the lower part of the Adriatic (Reka River) drainage is impossible because of the large altitudinal difference within. These results demonstrate that biological and particularly genetic groundwater tracing have the advantage of being sensitive to both historical and contemporary groundwater connectivity. Copyright (C) 2016 John Wiley & Sons, Ltd.