Glacial-driven vicariance in the amphipod Gammarus duebeni

We have examined the genetic diversity using mitochondrial COI and ND2 sequence data from 306 specimens of the amphi-Atlantic-distributed amphipod Gammarus duebeni. Marine populations from the Atlantic Ocean, the Baltic and North Sea, as well as freshwater populations from Ireland, Cornwall and Brittany were analysed. G. duebeni is a complex of five allopatric lineages. Freshwater populations result from multiple invasions of marine ancestors, represented by distinct lineages. We interpret the recent distribution of lineages as the outcome of a series of spatio-temporal vicariant events caused by Pleistocene glaciations and sea level changes. The freshwater lineages are therefore regarded as 'glacial relicts'. Furthermore, inter-specific competition with, for example, Gammarus pulex (which is absent in Ireland and western Brittany) may be another important determinant in the distribution of freshwater G. duebeni. In Ireland and Brittany, three freshwater refugia are suggested. The significantly limited gene flow detected among marine populations is more likely due to inter-specific competition than to salinity. The G. duebeni-complex represents a model system for the study of allopatric speciation accompanied by major habitat shifts. The pattern of spatio-temporal origins of the freshwater entities we describe here provides an excellent system for investigating evolutionary adaptations to the freshwater environment. Our data did not confirm the presently used subspecies classification but are only preliminary in the absence of nuclear genetic analyses.