The present position of Gammarus tigrinus Sexton, 1939, in the Netherlands, with the description of a newly discovered amphipod species, Crangonyx pseudogracilis Bousfield, 1958 (Crustacea, Amphipoda)

Since its first discovery in the Netherlands in 1964, Gammarus tigrinus has rapidly spread over inland waters in the western and north-eastern parts of the Netherlands. It appeared that at the end of 1976, G. tigrinus had replaced the local Gammarus species in most of the newly invaded waters. Surveys carried out in 1978 and 1979 showed a different and rather confusing pattern i.e that G. tigrinus is still extending its range in the delta region of the Netherlands and in the province of Zuid-Holland but that its spreading has come to a stand still in the rest of the country. It has even disappeared from a great number of formerly inhabited waters. In some of these waters indigenous species like G. d. duebeni and G. p. pulex reappeared after an absence of many years. The severe cold during the winter of 1978/1979 is considered to be one of the main causes of this decline of the G. tigrinus population. During the 1979 survey in the north-eastern part of the Netherlands, another, originally American, amphipod species, Crangonyx pseudogracilis Bousfield, 1958, was found. A description of this new faunal element is given.