| Summary: | The American amphipod species Melita nitida and Incisocalliope aestuarius have been found in the Western Scheldt estuary (the Netherlands). This is the first record of these species in the north-east Atlantic. Shipping is the most likely vector of introduction. The distribution of both species is investigated and compared with the distribution and the microhabitat of co-occurring amphipod species. Melita nitida is known from both the east and west coast of North America and I. aestuarius originates from the east coast of North America. Until now neither has been reported from other parts of the world. In the Netherlands both species are restricted to the mesohaline part of the Western Scheldt. Melita nitida occurs predominantly under Pacific oysters at the underside of boulders, mainly sublittorally. Incisocalliope aestuarius is associated to hydrozoans. Both microhabitats are hardly utilized by other amphipod species. Therefore, the theory that the existence of many empty niches in north-western European brackish waters make this environment particularly susceptible to invasions of alien species is corroborated. The application of hard substrates in a region originally predominated by soft bottoms moreover facilitates the introduction of exotic species. The species community on hard substrates in the mesohaline part of the Western Scheldt contains a high proportion of introduced species: approximately one third of the macrofauna species is of allochthonous origin.
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