Nieuwe gegevens over de verspreiding van de Amerikaanse zwaardschede Ensis directus (Conrad, 1843) in Europa
Severijns, N. (1999). Nieuwe gegevens over de verspreiding van de Amerikaanse zwaardschede Ensis directus (Conrad, 1843) in Europa. De Strandvlo 19(3): 126-140
In: De Strandvlo: Driemaandelijks Tijdschrift van De Strandwerkgroep België. De Strandwerkgroep België: Ursel. ISSN 0773-3542, more
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| Abstract |
Observations of the American jackknife clam Ensis directus (Conrad, 1843) that were carried out in the period 1992-1994 had shown that Ensis directus has settled in 1991 in Hardelot, about 10 km south of Boulogne-sur-Mer in the north of France (see map). During these observations the most southern place where Ensis directus was found was Quend-Plage. A similar study that was carried out since August 1996 indicates that at present the bay of the river Somme, about 55 kilometers south of Boulogne-sur-Mer is the most southern location in Europe where Ensis directus lives. The dataset shows that Ensis directus is present in the bay of the Somme since 1992 already. In seven years time it has built large populations in the whole region from Boulogne-sur-Mer to the bay of the Somme. In August 1996, specimens of Ensis directus were also found in Ault, about 10 kilometers south of the bay of the Somme. Their appearance, however, did not allow to determine whether these were washed ashore frorn the sublittoral region or whether they were brought there by the currents frorn near the bay of the Somme. However, since Ault is very close to the bay of the Somme, Ensis directus will most probably also settle in Ault. The only requirement for this is that the lower tidal region and the sublittoral region, where Ensis directus lives, contains enough fine sand mixed with silt and are not too rocky. It is indeed known frorn a previous study (Severijns & Vanhaelen, 1994) that Ensis directus does not settle on a rocky bottorn: between the Cap Griz Nez and Wimereux (see map) not a single observation of Ensis directus was reported until now. The species has nevertheless been able to bridge this distance of about 25 km due to its pelagic larval phase. It is not clear, however, whether Ensis directus will be able to continu to extend its area of distribution to the south. Indeed, frorn Ault southward the coastline is extremely rocky until the mouth of the river Seine near le Havre, a distance of about 150 km. It is therefore not inpossible that the region around the bay of the river Somme will turn out to be the most southern location of Ensis directus in Europe. Finally, it is observed that, similar to what previously have happened at other places in the region between Boulogne-sur-Mer and the bay of the river Somme also the populations of Ensis arcuatus are reduced in size significantly or even disappear at places where Ensis directus has settled. Only at Fort Mahon-Plage (see map) a rather large population of Ensis arcuatus is still present. Observations of Ensis directus frorn Denmark and Sweden are reported as well. These show that Ensis directus is also present in the sublittoral region of the Swedish coast. The length of the collected specimens ranged from 44 to 97 mm for those collected in March 1996 in south-west Sweden (on the sandy beach of Langasand, about midway between Falkenberg and Halmstad) and from 68 to 127 mm for the ones collected in June 1996 in Saeby, close to Frederikshaven in the very north of Denmark (see map). Note that specimens with a length of up to 60 mm only were previously reported from the Grönitz beach, near Lübeck in northern Germany (Wouters, 1995). These three observations indicate that in the northern part of its area of distribution Ensis directus does not become grow as large as is the case more south. Indeed, in Belgium and the north of France specimens with a length of about 170 mm are quite common. This length is usually reached after six to seven years after settlement of Ensis directus. It is known that Ensis directus has settled in the region of Frederikshaven in Denmark al ready since around 1982 (Essink, 1986a & 1986b), so that it certainly has had the time to reach dimensions up to 170 mm. The smaller size might be related to the salinity of the waters at the east coast of Denmark. Recent material of Ensis directus from the west coast of Denmark could shed more light on this. |
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