Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


Ensis directus



Ensis directus


  • Last modified
  • 06 November 2018
  • Datasheet Type(s)
  • Invasive Species
  • Preferred Scientific Name
  • Ensis directus
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Metazoa
  •     Phylum: Mollusca
  •       Class: Bivalvia
  •         Subclass: Heterodonta
  • Summary of Invasiveness
  • E. directus was first found in European waters in 1979 in the German Bight of the North Sea (von Cosel et al., 1982...

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Ensis directus; the American razor-clam or jack-knife clam.
TitleGeneral view
CaptionEnsis directus; the American razor-clam or jack-knife clam.
CopyrightJohan Craeymeersch
Ensis directus; the American razor-clam or jack-knife clam.
General viewEnsis directus; the American razor-clam or jack-knife clam.Johan Craeymeersch
Mass of Ensis directus
TitleEnsis directus
CaptionMass of Ensis directus
CopyrightJohan Craeymeersch
Mass of Ensis directus
Ensis directusMass of Ensis directusJohan Craeymeersch
Ensis directus shells deposited on the shore can be a nuisance to the public
TitleEnsis directus shells
CaptionEnsis directus shells deposited on the shore can be a nuisance to the public
CopyrightJohan Craeymeersch
Ensis directus shells deposited on the shore can be a nuisance to the public
Ensis directus shellsEnsis directus shells deposited on the shore can be a nuisance to the publicJohan Craeymeersch


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Preferred Scientific Name

  • Ensis directus (Conrad, 1843)

Other Scientific Names

  • Ensis americanus (Gould,1870)
  • Ensis arcuatus var directus (Jeffreys, 1865)
  • Solen directus Conrad, 1843
  • Solen ensis americana Gould & Binney, 1870

International Common Names

  • English: American jack knife clam; American razor-shell; razor clam

Local Common Names

  • : couteau Américain; couteau droit
  • Denmark: Amerikansk knivmusling
  • Germany: Amerikanische Scheidenmuschel; Amerikanische Schwertmuschel
  • Netherlands: Amerikaanse zwaardschede
  • Norway: Amerikaknivskjel

Summary of Invasiveness

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E. directus was first found in European waters in 1979 in the German Bight of the North Sea (von Cosel et al., 1982). The species originates from North American Atlantic waters and is thought to be transported via ballast water tanks of ocean crossing vessels.

E. directus has spread along the shores of Germany into Denmark (1981), Sweden (1982) and Norway (1989) as well as to the Netherlands (1981), Belgium (1985) and France (1991). From the early 1980s it spread all around the British Isles.

The species is remarkable in both the speed in which new areas are colonized and its dominance in abundance over all other shellfish species, causing competition for food and space. During a period of increase of E. directus the native shellfish Spisula subtruncata stock collapsed and has not recovered since. In several places E. directus has become the dominant species in the local ecosystem.

The species is fit for human consumption and due to its abundance new shellfish fisheries have developed in European waters.

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Metazoa
  •         Phylum: Mollusca
  •             Class: Bivalvia
  •                 Subclass: Heterodonta
  •                     Order: Veneroida
  •                         Unknown: Solenoidea
  •                             Family: Solenidae
  •                                 Genus: Ensis
  •                                     Species: Ensis directus

Notes on Taxonomy and Nomenclature

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Taxonomic revision of the genus Ensis by Van Urk (1964, 1972) suggested renaming the species Ensis directus to Ensis americanus, as the name E. directus was linked to an extinct Miocene species, originally described as Solen directus. Van Urk treated the recent specimens under the name Ensis americanus (Binney, 1870) without noticing that the name Solen americanus was preoccupied by Chenu (1843). The species is however more variable than was thought by Van Urk and differences cited by Van Urk fall within the variation of the recent form (von Cosel, 2009) and for this reason there is doubt if E. directus and E. americanus are really distinct species.

Therefore, the senior name E. directus is the appropriate name according to nomenclatorial rules. Also in conformity with the widely used name for the Western Atlantic species, the name E. directus is used.


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E. directus is a marine shellfish which is indigenous on the Atlantic East coast of the North American Continent where it is found from Labrador to South Carolina. The species is exotic in European waters and has colonized the sandy shores of the east Atlantic and North Sea from Norway to France. Its distribution may still be extended by further expansion of its range. During the early years expansion was slow but this accelerated during the 1990s.

The species is found in its native range in the west Atlantic as well as in its colonized area in the east Atlantic in the lower belt of the tidal zone and the upper zone of the sublittoral zone where fine sand is present. E. directus is buried in fine sand but the adult shell is able to move by itself for short distances. The larvae have a pelagic life stage by which they are transported by tidal waves and currents. Possibly this is the reason of their rapid dispersal during colonization.

Mass mortalities occur regularly in the European distribution range. This may be due to low water temperature.

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Sea Areas

Atlantic, NortheastPresentIntroduced Invasive Cosel Rvon, 2009First record from 1979
Atlantic, NorthwestPresentNative Not invasive Bousfield, 1960; Abbott and Morris, 1995From Labrador (Canada) to Florida

North America

CanadaPresentPresent based on regional distribution.
-New BrunswickPresentNative Not invasive Abbott and Morris, 1995
-Newfoundland and LabradorPresentNative Not invasive Abbott and Morris, 1995
-Nova ScotiaPresentNative Not invasive Abbott and Morris, 1995
-Prince Edward IslandPresentNative Not invasive Abbott and Morris, 1995
-QuebecPresentNative Not invasive Abbott and Morris, 1995
USAPresentPresent based on regional distribution.
-FloridaPresentNative Not invasive Abbott and Morris, 1995
-MainePresentNative Not invasive Abbott and Morris, 1995
-North CarolinaPresentNative Not invasive
-Rhode IslandPresentNative Not invasive Abbott and Morris, 1995
-South CarolinaPresentNative Not invasive Bousfield, 1960; Abbott and Morris, 1995


BelgiumWidespreadIntroduced Invasive Severijns, 2002
DenmarkWidespreadIntroduced Invasive Severijns, 2002
FranceWidespreadIntroduced Invasive Luczak and Dewarumez, 1992
GermanyPresent1979Cosel et al., 1982German bight of the North Sea
IrelandWidespreadSeverijns, 2002
NetherlandsPresentIntroduced Invasive Boer and Bruyne, 1983
NorwayPresentIntroduced Invasive Severijns, 2002
SwedenPresentIntroduced Invasive Jensen and Knudsen, 2005Most southern colonization in direction of the Baltic Sea
UKWidespreadIntroduced Invasive Severijns, 2002

History of Introduction and Spread

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E. directus was first found in Germany in 1979 and has probably made the transfer from the west Atlantic to the east Atlantic with ballast water of an ocean crossing vessel. From its first settlement in the German bight it found its way by natural distribution to other zones and is now found from Normandy in France to the south of Norway, including the British Isles.


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Introduced toIntroduced fromYearReasonIntroduced byEstablished in wild throughReferencesNotes
Natural reproductionContinuous restocking
Germany 1979 Yes No Cosel et al. (1982)

Risk of Introduction

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E. directus is both in number and biomass an abundant species and for that reason dominates the local fauna in its range. Competition for space and food may be the result of a changed abundance of the local indigenous species. This is likely the case for its impacts upon Spisula subtruncata.

As the species is both a food source for shellfish-eating sea birds and is exploited for human consumption the species may not be regarded as a pest as such but perhaps as an interesting new natural resource.

Habitat List

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Estuaries Present, no further details
Inshore marine Present, no further details
Benthic zone Present, no further details

Biology and Ecology

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Species-specific restriction endonuclease patterns were found with the enzymes HaeIII for E. arcuatus, E. siliiqua and E. directus, and AcsI for E. macha, and when two enzymes were combined, all four species could be identified (Fernandez-Tajes and Mendez, 2007). Vierna et al. (2010) report on the use of ITS1 and ITS2 sequences to distinguish Ensis at the both the species and population level.

Reproductive Biology

Reproduction takes place in the first half of the year and larvae have a free floating pelagic stage before settling. Postlarvae settle themselves in May to June. Colonization is often massive.

Water Tolerances

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ParameterMinimum ValueMaximum ValueTypical ValueStatusLife StageNotes
Depth (m b.s.l.) Optimum Tolerates the lower intertidal to 30 m

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Cerebratulus lacteus Predator Adults
Haematopus ostralegus Predator Adults to species
Larus Predator Adults to genus
Larus delawarensis Predator Adults to species
Polinices heros Predator Adults to species

Notes on Natural Enemies

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In the eastern Atlantic, natural enemies also include shellfish-eating ducks.

Economic Impact

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E. directus is fished for human consumption.

Threatened Species

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Threatened SpeciesConservation StatusWhere ThreatenedMechanismReferencesNotes
Spisula subtruncataNo details No detailsCompetition - monopolizing resources

Social Impact

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The presence of dead shells from E. directus on beaches is a nuisance for bare-footed beach walkers.

Mechanical removal of dead shells is occasionally carried out.


Risk and Impact Factors

Top of page Invasiveness
  • Proved invasive outside its native range
  • Abundant in its native range
  • Pioneering in disturbed areas
  • Fast growing
  • Has high reproductive potential
  • Gregarious
Impact outcomes
  • Ecosystem change/ habitat alteration
  • Modification of natural benthic communities
  • Monoculture formation
  • Reduced native biodiversity
Impact mechanisms
  • Competition
  • Filtration
  • Herbivory/grazing/browsing
  • Predation
  • Rapid growth
Likelihood of entry/control
  • Highly likely to be transported internationally accidentally
  • Difficult/costly to control

Prevention and Control

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Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.


Ballast water, a known vector of E. directus, is the subject of many national and international initiatives focussing on reducing its risk as a transport vector of invasive species.


There is no management of introduced E. directus other than the issuing of a limited number of fishing licenses.



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Abbott RT, Morris PA, 1995. Shells of the Atlantic and Gulf Coasts and the West Indies. Boston: Peterson Field Guides, Houghton Mifflin Company.

Boer TWde, Bruyne RHde, 1983. [English title not available]. (De Amerikaanse zwaardschede Ensis directus (Conrad, 1843) in Nederland.) Basteria, 47:154.

Bousfield EL, 1960. Canadian Atlantic sea shells. Canadian Atlantic sea shells. Ottowa: Dept. of Northern Affairs and National Resources, National Museum of Canada, 72 pp.

Cosel Rvon, 2009. The razor shells of the eastern Atlantic, part 2. Pharidae II: the genus Ensis Schumacher, 1817 (Bivalvia, Solenoidea). Basteria, 73:9-56.

Cosel Rvon, Dörjes J, Mohlenhardt-Siegel U, 1982. [English title not available]. (Die amerikanische Schwertmuschel Ensis directus (Conrad) in der Deutschen Bucht. I. Zoogeographie und Taxonomie im Vergleich mit den einheimischen Schwertmuschel-Arten.) Senckenbergiana Maritime, 14:147-173.

Fernández-Tajes J, Méndez J, 2007. Identification of the razor clam species Ensis arcuatus, E. siliqua, E. directus, E. macha, and Solen marginatus using PCR-RFLP analysis of the 5S rDNA region. Journal of Agricultural and Food Chemistry, 55(18):7278-7282.

Jensen KR, Knudsen J, 2005. A summary of alien marine benthic invertebrates in Danish waters. Oceanological and Hydrobiological Studies, XXXIV(Supplement 1):137-162.

Kerckhof F, Haelters J, Gollasch S, 2007. Alien species in the marine and brackish ecosystem: the situation in Belgian waters. Aquatic Invasions, 2(3):243-257.

Luczak C, Dewarumez J-M, 1992. Note on the identification of Ensis directus (Conrad, 1843). Cahier de Biologie Marine, 33:515-518.

Severijns N, 2002. Distribution of the American jack-knife clam Ensis directus (Conrad, 1843) in Europe, 23 years after its introduction. (Verspreiding van de Amerikaanse zwaardschede Ensis directus (Conrad, 1843) in Europa 23 jaar na de introductie: opmerkelijke opmars van een immigrant.) Gloria Maris, 40:63-111.

Severijns N, 2004. New notes on the distribution of Ensis directus (Conrad, 1843) in Western Europe. Gloria Maris, 43:19-30.

Vierna J, Martinez-Lage A, Gonzalez-Tizon AM, 2010. Analysis of ITS1 and ITS2 sequences in Ensis razor shells: suitability as molecular markers at the population and species levels, and evolution of these ribosomal DNA spacers. Genome, 53(1):23-34.

Links to Websites

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Alien species in the Belgian part of the North sea and adjacent estuaries
DAISIE Delivering Alien Invasive Species Inventories for Europe
Research project: Ecosystem sensitivity to invasive species (EnSIS)


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21/12/10 Original text by:

Johan Craeymeersch, Institute for Marine Resources and Ecosystem Studies, Wageningen IMARES, Wageningen UR, Netherlands

Kees Goudswaard, Institute for Marine Resources and Ecosystem Studies, Wageningen IMARES, Wageningen UR, Netherlands


Distribution Maps

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