The position of two introduced amphipod crustaceans, Gammarus tigrinus and Crangonyx pseudogracilis in the Netherlands during the period 1987-1988

During two and a half decades periodic survey programs have been carried out to follow the dispersal of Gammarus tigrinus. Since the last survey in the period 1985-1986 the species consolidated its position and showed a minor range extension in the southeastern part of the country. Gammarus specimens from the island Terschelling were electrophoretically investigated and compared with known G.tigrinus and G.zaddachi from the mainland. These experiments demonstrated the presence of G.tigrinus on this island. Crangonyx pseudogracilis which showed some range extension in the 1985-1986-survey, again was found outside its hitherto known distribution area.