The amphipod, Gammarus tigrinus Sexton, 1939, introduced in the Netherlands (Crustacea)

An alien amphipod, Gammarus tigrinus Sexton, has been observed in the Netherlands, mainly in oligohaline waters. It has had an “explosive” development in the Yssellake (the former Zuydersea) and occurs also in a great number of inland waterways in the province of North-Holland (fig. 1). It co-exists with, and often outnumbers, the local gammarids, G. pulex, G. zaddachi and G. duebeni. It is expected that the extension of its range will continue next years; fig. 2 illustrates the range of this introduced species in autumn 1965. Since Irish specimens of G. tigrinus were released in 1960 in the Yssellake, it is not clear whether the Dutch populations originate from the Irish stock, or whether that they were accidentally, e.g. in ballast tanks, carried to the Netherlands by ships coming from Germany, where a population explosion has taken place recently in the Weser-system.