First Record of the American Jack Knife ClamEnsis Directuson the French Coast of the North Sea

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Abstract
Ensis directus(Bivalvia: Solenidae) was noted for the first time along the French coast of the North Sea in June 1991. High numbers of post larvae after a period of northerly winds indicate that settled larvae originated from adult populations in Belgian or Dutch coastal waters. Future dispersal ofEnsis directuscould be used as a biological tracer of coastal water movements between the Southern Bight of the North Sea and the English Channel.The American jack knife clam,Ensis directus(Conrad, 1843), synonymEnsis americanus(Binney, 1870)sensuvan Urk (1964,1972), was discovered in Europe in the German Bight for the first time in June 1979 (Von Coselet al., 1982). This species is assumed to have been transported in its larval stage by a ship containing ballast water. Considering the dimensions of the specimens, this probably happened in the first half of 1978 (Von Coselet al., 1982; Mühlenhardt-Siegelet al., 1983). Since then this species has spread rapidly in the North Sea in subtidal and intertidal areas. Dense populations were found along the German coast within a few years (Von Coselet al., 1982; Mühlenhardt-Siegelet al., 1983; Swennenet al., 1985). In 1986 Ensisdirectuswas reported from the north and east Danish coasts as far as the Belgian coast (Kerkhof & Dumoulin, 1987) (Figure 1). Since then no new records have been reported.