Ensis directus

Pictures

Picture	Title	Caption	Copyright
Title General view Caption Ensis directus; the American razor-clam or jack-knife clam. Copyright Johan Craeymeersch	General view	Ensis directus; the American razor-clam or jack-knife clam.	Johan Craeymeersch
Title Ensis directus Caption Mass of Ensis directus Copyright Johan Craeymeersch	Ensis directus	Mass of Ensis directus	Johan Craeymeersch
Title Ensis directus shells Caption Ensis directus shells deposited on the shore can be a nuisance to the public Copyright Johan Craeymeersch	Ensis directus shells	Ensis directus shells deposited on the shore can be a nuisance to the public	Johan Craeymeersch

Identity

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

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

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). 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

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

Notes on Taxonomy and Nomenclature

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.

Distribution

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

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.

History of Introduction and Spread

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.

Introductions

Risk of Introduction

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

Biology and Ecology

Genetics

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

Natural Enemies

Notes on Natural Enemies

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

Pathway Causes

Pathway Vectors

Economic Impact

E. directus is fished for human consumption.

Threatened Species

Social Impact

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

Impact mechanisms

• Competition
• Filtration
• Herbivory/grazing/browsing
• Predation
• Rapid growth

Impact outcomes

• Ecosystem change/ habitat alteration
• Modification of natural benthic communities
• Monoculture formation
• Reduced native biodiversity

Invasiveness

• Abundant in its native range
• Fast growing
• Gregarious
• Has high reproductive potential
• Pioneering in disturbed areas
• Proved invasive outside its native range

Likelihood of entry/control

• Difficult/costly to control
• Highly likely to be transported internationally accidentally

Prevention and Control

Prevention

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.

Control

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

References

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 , 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.

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.

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. http://pubs.acs.org/journals/jafcau/index.html

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. http://www.aquaticinvasions.ru/2007/AI_2007_2_3_Kerckhof_etal.pdf

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.

Links to Websites

Contributors

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

• = Present, no further details
• = Evidence of pathogen
• = Widespread
• = Last reported
• = Localised
• = Presence unconfirmed
• = Confined and subject to quarantine
• = See regional map for distribution within the country
• = Occasional or few reports

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