The distribution of two common intertidal amphipod species Gammarus oceanicus and Gammarus setosus was studied along the coast of Svalbard Archipelago. Genetic analysis showed geographical homogeneity of G. oceanicus with only one molecular operational taxonomic unit (MOTU) and much higher diversifcation of G. setosus (5 MOTUs) in the studied area. Only two MOTUs of G. setosus are widespread along the whole studied Svalbard coastline, whereas the remaining three MOTUs are present mainly along the northern and eastern parts of archipelago’s largest island, Spitsbergen. Distribution analysis indicates that the demographic and spatial expansion of G. oceanicus in the northern Atlantic has started already during the Last Glacial Maximum (LGM, ca. 18 ka), while G. setosus seems to be a long-persistent inhabitant of the Arctic, possibly even through the LGM, with slower distribution dynamics. Combining the results of our molecular study with previous feld observations and the knowledge upon the direction of ocean currents around the Svalbard Archipelago, it can be
assumed that G. oceanicus is a typical boreal Atlantic species that is still continuing its postglacial expansion northwards.
In recent decades it colonized High Arctic due to the climate warming and has partly displaced G. setosus, that used to be the only common gammarid of the Svalbard intertidal zone.