De Amerikaanse zwaardschede Ensis directus (Conrad, 1843) nu ook in Normandië
Severijns, N. (2000). De Amerikaanse zwaardschede Ensis directus (Conrad, 1843) nu ook in Normandië. De Strandvlo 20(2): 47-53
In: De Strandvlo: Driemaandelijks Tijdschrift van De Strandwerkgroep België. De Strandwerkgroep België: Ursel. ISSN 0773-3542, meer
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| Abstract |
On the afternoon of 23- VIII-1999, a total of 23 doublets of Ensis directus werecollected on the beach in front of the "Falaises des Vaches Noirs" in Villers-sur-Mer, about 20 km south of Le Havre (France ). Previously, Ault, some 10 km south of the bay of the river Somme, was the most southern location in western Europe were Ensis directus was found. This is about 160 km more north than Villers-sur-Mer. Since all collected specimens have a fresh to very fresh appearance (glossy periostracum) it is concluded that Ensis directus lives in Villers-sur-Mer. The length of the collected specimens (i.e. up to 150 mm and according to the growth lines in their fourth year) indicates that Ensis directus arrived there in 1996. When extending its area of distribution southward Ensis directus moves in the direction opposite to that of the normal, northward-oriented, sea currents in this area. Large distances can be covered only during the pelagic phase. Previously it was found, however, that larvae and post-larvae of Ensis directus are available during the larger part of the year (Essink, 1985). Moreover, periods of up to several weeks with predominantly northerly to northeasterly winds, allowing for transport in the direction opposite to the normal currents, and this, in fact, throughout the entire year (Essink, 1986). During such periods distances of up to several tens of kilometres can be covered (Essink, 1985). Ensis directus has covered the distance of about 170 km between the bay of the Somme and that of the Seine in about four years time. Previously, six years were needed to cover the slightly shorter distance between the Belgian coast and the bay of the Somme. North of the bay of the Somme most beaches are sand beaches where Ensis directus has no difficulties to find its preferred habitat, i.e. a sand/mud bottorn at the border of the littoral and sublittoral region or in the upper sublittoral. However, south of the bay of the Somme the shore is extremely rocky and only a limited number of locations suitable for Ensis directus. Since this long distance with predominantly rocky coasts was covered much faster than the shorter distance with predominantly sandy coasts between the Belgian coast and the bay of the Somme, one has to conclude that the rocky shores were not capable at aIl of slowing down the distribution of Ensis directus towards the south. A second, and probably even more important conclusion is that the speed at which Ensis directus can extend its area of distribution to the south is determined primarily by the occurrence of N- and NE-winds and not by the topology of the coasts ! The long distance of about 170 km between the bay of the Somme and ViIlers-sur- Mer was most probably not covered in one time, but rather in several stages, possibly even spread over a period of several years. This would then mean that populations of Ensis directus are present between the bay of the Somme and the bay of the Seine. It would be interesting if evidence for such popu1ations would be found, as this would prove this scenario. South of the bay of the river Seine sand beaches are very abundant again. Since Ensis directus arrived in ViIlers-sur-Mer as early as 1996, it has in the mean time very probably already settled at places more south as well. It therefore remains interesting to continue monitoring the spreading of this species. |
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