Rapid expansion and facilitating factors of the Ponto-Caspian invader Dikerogammarus villosus within the eastern Baltic Sea

Dikerogammarus villosus, an amphipod of Ponto-Caspian origin, has recently and rapidly spread along Baltic coastal lagoons and estuaries. By 2016 it had invaded Russian (Kaliningrad region), Lithuanian and Latvian waters, but was not recorded from Estonian waters. This species has a discontinuous distribution suggesting a “jump” was involved in its dispersal. A classification tree and GLM analyses confirm such an observed distribution pattern and suggest productivity of the environment, distance to the nearest lagoon/river mouth and distance to the nearest port/marina were the most influential explanatory variables of its distribution. Our data indicates this rapid east and northward expansion is very likely due to vessel transport, which would account for the “jump” dispersal. Other vectors facilitating further spread are almost certainly acting at a local scale such as overland transportation of vessels, movements of diving gear, drifting mats of algae, macrophytes and flotsam, as well as natural spread. We predict the “killer shrimp” will soon appear within the entire Gulf of Riga and the Gulf of Finland, and also expand up-rivers of the eastern Baltic Sea. Following the species expansion, alterations and changes to macroinvertebrate assemblages in invaded areas can be expected