Ensis americanus


Phylum: Mollusca
Class: Pelecypoda
Order: Veneroida
Species name: Ensis americanus (Gould in Binney 1870), following the taxonomy of van Urk (See 1964 & 1972 references in Urk (1987).
Synonyms: Ensis directus auctt. non Solen directus (Conrad 1843)
Common name: American jack knife clam
Date of introduction and origin
Ensis americanus was found in 1989 on Holme beach, Norfolk (Howlett 1990). It is native to the Atlantic coast of North America and is thought to have been introduced from there, probably via mainland Europe (K. Essink pers. comm.).
Method of introduction
It was supposedly introduced to Europe as larvae in tanker ballast water; its spread within European waters has been by pelagic larvae (Cosel, Dörjes & Mühlenhardt-Siegel 1982).
Reasons for success
Rate of spread and methods involved
It has spread rapdily in southern North Sea countries (Essink 1985). Ensis americanus spread from its point of introduction in the German Bight in 1978 (Cosel, Dörjes & Mühlenhardt-Siegel 1982) around the North Sea coast of Denmark and The Netherlands (Essink 1985) by 1982, to Belgium by 1984 (Essink 1986) and France by 1986. It had reached the English Channel by the end of the 1980s (see Luczak, Dewarumez & Essink (1993) for map). The pelagic larval stage is assumed to be transported with water currents (Cosel, Dörjes & Mühlenhardt-Siegel 1982).
Currently it is found at sites along the British east coast south from the Humber and along the English Channel west as far as Rye Harbour, East Sussex. (Howlett 1990; J. Light & I. Killeen pers. comm.). It is common in the Wash (R. Bamber pers. comm.). Elsewhere in Europe it is found in the Low Countries (Boer 1984 and see above).
Factors likely to influence spread and distribution
It has long-lasting pelagic larvae.
Effects on the environment
Effects on commercial interests
Control methods used and effectiveness
None used.
Beneficial effects
It is fished in some parts of continental Europe.
In some places, e.g. Southend on Sea, Essex, in 1995 it was reported to be one of the commonest living bivalves on the shore (J. Light & I. Killeen pers. comm.).
Boer, Th.W. de., 1984. The American jack knife clam Ensis directus (Conrad, 1843) in Europe. Bolletino Malacologica (Milano), 20: 258-262.
Cosel, R. von, Dörjes, J., & Mühlenhardt-Siegel, U. 1982. Die Amerikanische Schwertmuschel Ensis directus (Conrad) in der Deutschen Bucht. I. Zoogeographie und Taxonomie im Vergleich mit den einheimischen Schwertmuschel-Arten. Senckenbergia Maritima, Frankfurt-am-Main, 14: 147-173.
Essink, K. 1985. On the occurrence of the American jack knife clam Ensis directus (Conrad, 1843) (Bivalvia: Cultellidae) in the Dutch Wadden Sea. Basteria, 49: 73-80.
Essink, K. 1986. Note on the distribution of the American jack knife clam Ensis directus (Conrad, 1843) in NW Europe (Bivalvia: Cultellidae). Basteria,50: 33-34.
Howlett, D.J. 1990. The arrival in Britain of Ensis americanus. Conchologist's Newsletter, No. 114: 301-302.
Luczak, C., Dewarumez, J.M., & Essink, K. 1993. First record of the American jack knife clam Ensis directus on the French coast of the North Sea (short communication). Journal of the Marine Biological Association of the United Kingdom, 73: 233-235.
Urk, R.M. van., 1987. Ensis americanus (Binney) (syn. E. directus auct. non Conrad) a recent introduction from Atlantic North America. Journal of Conchology, 32: 329-333.
Dr K. Essink, National Institute for Coastal and Marine Management, The Netherlands.