A shrimp between two worlds: the genetic differentiation of the brackish water shrimp Palaemon varians Leach, 1813 in the Baltic and the North Sea

The patchy distribution of the ditch shrimp Palaemon varians (Palaemonidae) in small-scale habitats with low salinity may hint at isolation and fragmentation processes. In order to evaluate the extent to which gene flow barriers exist and salinity has an influence on this dispersal pattern, the genetic diversity and differentiation of Baltic and North Sea populations were investigated and set in context with sequence data from the Mediterranean Sea. Therefore, 133 sequences of a fragment of the cytochrome oxidase subunit 1 (COI) from 13 different populations from the Baltic Sea and North Sea were generated and complemented with publicly available sequences to estimate a haplotype network and calculate genetic differentiation and diversity. The results show that genetic differentiation correlates with geographic distance and the populations of the different basins are differentiated from each other. In contrast to the expectation, genetic differentiation was generally weak within basins. However, different clusters of differentiated populations could be revealed, indicating two migration events into the Baltic Sea.