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De verspreiding van Ensis directus (Conrad 1843) in Europa, 23 jaar na de introductie: de opmerkelijke opmars van een immigrant
Severijns, N. (2001). De verspreiding van Ensis directus (Conrad 1843) in Europa, 23 jaar na de introductie: de opmerkelijke opmars van een immigrant. De Strandvlo 21(4): 123-146
In: De Strandvlo: Driemaandelijks Tijdschrift van De Strandwerkgroep België. De Strandwerkgroep België: Ursel. ISSN 0773-3542, more
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Keywords
    Distribution; Introduced species; Ensis directus (Conrad, 1844) sensu Abbott, 1954 [WoRMS]; Atlantic North East [Marine Regions]; ANE, Belgium [Marine Regions]; Marine

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  • Severijns, N., more

Abstract
    The distribution of the American jack-knife clam Ensis directus (Conrad, 1843) (photo 1, figure 1,2) in Europe is discussed. The first specimens of this North- American species were found in 1979 in the German Bight, near the mouth of the river Elbe (Von Cosel et al., 1982). Most probably larvae were transported to Europe in 1978 with tanker ballast water. At present, about 23 years later, E. directus has spread considerably along the Western-European coasts. Towards the north (table 2, figure 3) it is found up to Oslo in Norway, has spread all around the Danish coast up to the south-western part of the Ostsea and occurs also along the west coast of Sweden. Southward (table 1, figure 3) it is found on the beaches of The Netherlands and Belgium, in the south-eastern part of England (from the Humber estuary southward along the east coast to Newhaven at the English channel coast (Eno, 1998) and along the French coast as far south as Merville-Franceville (south of Le Havre) in Normandy (Severijns, 2000a, 2000b). Most probably the species will continue to further extend its area of distribution, as in different parts of it suitable habitats are within reach. The possible influence of E. directus on the occurrence of the local European Ensis and Solen species was discussed. Observations indicate that E. arcuatus (Jeffreys, 1865) and E. minor (Chenu, 1843) do not occur any more at the Belgian coast. Indeed, no significant quantities of fresh specimens have been found on Belgian beaches since 1997. Before, both species were very common and until 1997 relatively large amounts (for E. arcuatus even up to several hundred specimens at a time) were regularly washed ashore (e.g. Rappé, 1985, Severijns, 1994, 1996; Vanhaelen, 1997a, 1997b, 1998). Solen marginatus, has never been a common species at the Belgian coast, but from time to time, especially af ter several consecutive soft winters, populations of Solen marginatus do occur. In the past 15 years three strandings occurred: in the period April-September 1985 (several tens of fresh specimens collected; Rappé, 1985; Kerckhof, 1987), from November 1992 to March 1993 (about 50 fresh specimens collected; Vanhaelen, 1993b), and in 1997 (more than 800 fresh specimens collected; Vanhaelen, 1997b, 1999). Since E. directus is present at the Belgian coast since 1985, these strandings, and especially the very large stranding in 1997, indicate that Solen marginatus is not affected by the presence of E. directus. At the French coast E. arcuatus was strongly reduced in numbers or may even have disappeared also at most places between the Belgian-French border and the bay of the Somme. However, near Fort-Mahon significant numbers of fresh specimens of E. arcuatus can still be found, while also Solen marginatus (Pulteney, 1799) and E. ensis (Linnaeus, 1758) are still present there, albeit in smaller amounts (Severijns, 1999). In Villers.sur-Mer in Normandy, where E. directus arrived in 1996, E. arcuatus, E. ensis and Solen marginatus are also still present and their numbers seem not to have decreased (Severijns, 2000a, 2000b). It thus seems that the disappearance of E. minor and E. arcuatus in Belgium and the strong reduction or disappearance of E. arcuatus in the region north of the bay of the Somme in France is not re1ated to the presence of E. directus there. Also in British waters there is, at far as known, no evidence that E. directus is competing with other species of razor clams (Eno, 1998). However, more observations and study about this would be very useful.

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