Range extension in the period 1985-1986 of the alien amphipods, Gammarus tigrinus, 1939, and Crangonyx pseudogracilis Bousfiled, 1958, in the Netherlands (Crustacea, Amphipoda)

Since its first discovery in the Netherlands in 1964, the dispersal of Gammarus tigrinus over inland waters is followed closely. In a survey carried out in 1985 and 1986 it appeared that G. tigrinus has extended its range in the southwestern and southeastern parts of our country. In the rest of the country the spreading has come to an apparent standstill. Apart from the mainland, G. tigrinus has been found for the first time on some Frisian isles: Texel, Terschelling and Ameland. During the present study the position of another alien amphipod, Crangonyx pseudogracilis, was investigated as well. For the first time some range extensions were found, since its initial discovery in 1979, in a limited area in the province of Groningen. Some remarks are made on the position of the indigenous Gammarus species.