Short description of Ensis leei, American jack knife clam

A thin, elongated bivalve (the shell has two parts) reaching lengths of 20 cm.  The length is about six times the width, and both tips are equally wide.  Pink to purple-brown bands on a yellow to red-brown background, with many growth bands evident.

Impact summary: Ensis leei, American jack knife clam

Impacts are uncertain; some see fishing opportunities, others fear a strong ecological impact on native species. The American jack knife clam inhabits a niche unoccupied by native clams, in brackish waters such as estuaries, whereas native razor clam species live in fully marine habitats. 

Habitat summary: Ensis leei, American jack knife clam

The American jack knife clam inhabits sand or muddy sand in the lower intertidal and shallow subtidal zone of marine and brackish water environments.

Overview table

Environment	Marine
Species status	Non-Native
Native range	Northeastern U.S.A.
Functional type	Filter-feeder
Status in England	Non-Native
Status in Scotland	Non-Native
Status in Wales	Non-Native
Location of first record	Southend on Sea
Date of first record	1989

Origin

The American jack knife clam originates from the east coast of North America, and is distributed from Labrador (Canada) to Florida.

First Record

The American jack knife clam was first recorded in Europe in the German Bight in 1978, and spread rapidly throughout the Wadden Sea during the 1980s.  It had reached the English Channel by the late 1980s and was found in abundance in the Wash during dredging in 1997.

Pathway and Method

The American jack knife clam is presumed to have arrived in Europe in the ballast water of a ship crossing the Atlantic.  The larvae are pelagic for up to four weeks, facilitating transport by natural dispersal and human activity.  In Holland, it is thought to have been unintentionally transported with mussel seed for aquaculture.  The precise method of introduction into GB is unknown.

Species Status

Now common from northern Denmark to northern France on the continental North Sea coast, and from the Humber estuary to Kent on the east coast of England.  Grab sampling in the Wash and North Norfolk candidate Special Area of Conservation (cSAC) in 2004 found the American jack knife clam to be by far the most common species of razor clam.  Following introduction to the German Bight in 1978 it spread rapidly along the North Sea coastline, travelling about 500 km within a three to four year period. 

Dispersal Mechanisms

The four week, free swimming pelagic larval period enables dispersal along the coast by water currents or further afield within the ballast water of ships.  Post-larval juveniles are also able to swim or use byssal threads for drifting (analogous to the gossamer flight of young spiders). 

Reproduction

Spawning periods are thought to occur twice yearly, in April/May and August/September.  Sexes are separate; eggs and sperm are released through siphons to be fertilized externally.  Larvae are pelagic for four weeks before settlement occurs on sand/mud substrate.  The American jack knife clam has a short life span (< seven years) compared to the native Ensis siliqua which lives for up to 19 years.

Known Predators/Herbivores

The American jack knife clam is preyed upon by a variety of birds such as oystercatcher, common scoter and herring gull.  In its native range it is also eaten by starfish and certain worms.

Resistant Stages

None known.

Habitat Occupied in GB

Inhabits a range of substrates from almost pure mud on the banks of channels within The Wash, to clean sand on the North Norfolk coast.  Within these areas the American jack knife clam occupies lower parts of the intertidal and shallow subtidal.  In contrast to the native species, which populate fine sand sediments in full salinity, the American jack knife clam inhabits a range of sediments from mud through silt to quite coarse sand and can tolerate more estuarine conditions.

Native range extends along the Atlantic coast of North America, from Labrador (Canada) to Florida.   In GB the American jack knife clam is common along the East coast from Humber to Kent, also found in other southern areas of the Channel and recorded locally near Milford Haven, south Wales. 

Environmental Impact

Although the American jack knife clam has successfully established large populations and it has been suggested that such abundance may have the potential to significantly alter local communities, no direct impacts on native species or communities have been reported.   Effects on local sediment structure may arise where dense populations occur, due to burrowing activity.   

Health and Social Impact

Injuries may be sustained from stepping on the sharp edge of the shell while walking barefoot.  Similar injuries can occur with native razor clams, but the American jack knife clam lives at shallower depths and as such poses a greater risk. 

Economic Impact

No economic impact has been reported, although the American jack knife clam may damage trawling nets on the seabed causing economic losses to fisheries.  This species is exploited commercially in GB waters.

Identification

von Cosel, R. & Gofas, S. (2010) Ensis directus (Conrad, 1843). In: Bouchet, P.; Gofas, S.; Rosenberg, G. (2010) World Marine Mollusca database. Accessed through: World Register of Marine Species.  Available from: http://www.marinespecies.org/aphia.php?p=taxdetails&id=140732

Biology, ecology, spread, vectors

Armonies, W. (2001) What an introduced species can tell us about the spatial extension of benthic populations. Marine Ecology Progress Series, 209, 289-294.

Cardoso, J.F.M.F., Witte, J.I. & Van Der Veer, H.W. (2009) Reproductive investment of the American razor clam Ensis americanus in the Dutch Wadden Sea. Journal of Sea Research, 62, (4), 295-298.

Management and impact

Beukema, J.J. & Dekker, R. (1995) Dynamics and growth of a recent invader into European coastal waters: the American razor clam, Ensis directus. Journal of the Marine Biological Association of the United Kingdom, 75, (2), 351-362.

Palmer, D. (2003) The introduced razor fish Ensis directus in The Wash and North Norfolk. Shellfish News, 16, 13-15.

General

Palmer, D.W. (2004) Growth of the razor clam Ensis directus, an alien species in the Wash on the east coast of England. Journal of the Marine Biological Association of the United Kingdom, 84, (5), 1075-1076.