Ensis americanus
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
Unknown.
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).
Distribution
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
Unknown.
Effects on commercial interests
Unknown.
Control methods used and effectiveness
None used.
Beneficial effects
It is fished in some parts of continental Europe.
Comments
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.).
References
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.
Acknowledgements:
Dr K. Essink, National Institute for Coastal and Marine
Management, The Netherlands.