The unclosed side is turned up when burrowed into the substrate, mostly sand or muddy sand, in which it forms a hole. Goes up and down into the substrate with a respectable speed (disappears into sand within 15 seconds).

Ensis directus is found along the Atlantic coast from Canada to South Carolina. It lives in the intertidal zone or subtidal zone in the sand or muddy bottoms.

(Jobin and Jobin 1997, Gosner 1978)

Biogeographic Regions: atlantic ocean (Native )

A recent immigrant from America, this species is now very common along the European coasts. First discovered on German North Sea Coasts in 1979, it had reached NE Denmark and the Dutch Wadden Sea by 1982, and NE France by 1991.

Labrador to Florida

Atlantic Europe, Bay of the Somme, Belgian Exclusive Economic Zone, Boulogne-sur-Mer, Cobscook Bay, Danish Exclusive Economic Zone, De Haan, De Panne, Dunkerque, Dutch Exclusive Economic Zone, European waters (ERMS scope), German Bight, Gulf of Maine, Gulf of Mexico, Manicouagan, North East Atlantic, North West Atlantic, Oostduinkerke, Saint Lawrence River, St. Lawrence Estuary, Wimereux, Zeebrugge

Ensis directus has a thin, elongated shell that is slightly curved. It ranges from yellowish to dark brown in color. The length of E. directus is about six times its width. It can grow to be about 10 in. It has a coating around its shell to protect it from eroding in the mud or sand. Ensis directus is a bivalve, which means that its shell has two parts. The body of Ensis directus is surrounded by the mantle and the mantle is seperated into two parts. Each part of the mantle secretes a shell. The two shells are connected by an elastic ligament that allows for it to open and close. Both parts are usually identical and are made up of calcium carbonate and protein. It has a huge foot that allows it to move through water or to burrow in the sand. When the foot is extended all the way, it is almost as long as the clam's body.

(Alexander 1979; Lippson 1984)

Whitish-grey mollusc with brown drawings. Length six times as wide, slightly curved, and can attain at least 20 cm long. Both tips equally wide.With many growing bands and a scaly surface. As with all Ensis spp. one side is never completely closed.

Conrad, 1843. Proc. Acad. nat. Sci. Philadelphia, 1: 325

Ensis directus lives in the sandy bottoms in the intertidal or subtidal zones along the Atlantic coast. It is usually found in colonies. It is not migratory and therefore it remains in its habitat year round.

(Gosner 1978)

Aquatic Biomes: coastal

infralittoral and circalittoral of the Gulf and estuary

In sand or muddy sand.

Depth range based on 1945 specimens in 1 taxon.
Water temperature and chemistry ranges based on 732 samples.

Environmental ranges
  Depth range (m): -99 - 260
  Temperature range (°C): 6.054 - 25.634
  Nitrate (umol/L): 0.289 - 13.639
  Salinity (PPS): 32.282 - 36.231
  Oxygen (ml/l): 4.099 - 6.764
  Phosphate (umol/l): 0.093 - 0.963
  Silicate (umol/l): 0.756 - 7.673

Graphical representation

Depth range (m): -99 - 260

Temperature range (°C): 6.054 - 25.634

Nitrate (umol/L): 0.289 - 13.639

Salinity (PPS): 32.282 - 36.231

Oxygen (ml/l): 4.099 - 6.764

Phosphate (umol/l): 0.093 - 0.963

Silicate (umol/l): 0.756 - 7.673
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

The American jack knife clam Ensis directus originates from the American east coast, and came to Europe through transport (of its larvae) in ballast water of cargo ships. In 1987, the first shells of the American jack knife clam were found on the beach of Oostduinkerke. Soon, the whole coastline was populated. The American jack knife clam is an opportunistic species. The impact on the ecosystem is twofold. Local economy can benefit from this species, as it can be fished and consumed, although the clam is also known to damage fishing nets. Although the species may serve as food for sea birds, its also feared that the American jack knife clam might negatively influence biodiversity.

Ensis directus is a filter feeder that filters water through its shell in order to obtain food. When feeding, E. directus stays very close to the surface and its siphons are sticking up through the surface. The water is drawn into the shell through the mantle cavity by cilia. These cilia cover the ctenidia, or gills, in the clam. It passes along the gills and combines with mucous. The food is now trapped and the cilia drive the food into the digestive tract.

(Jobin and Jobin 1997)

There are separate male and female sexes in Ensis directus. The males release their sperm into the water and the sperm enters the female through openings. The eggs are fertilized in the interior of the gill by the sperm and these newly fertilized zygotes develop into larva. This larva is then released into the surrounding water. There are two larval stages. The first stage is the trocophore stage that has small larvae that are free swimming. They are pear shaped, translucent, and ciliated. The second stage is the veliger stage, which is also a free-swimming larval stage. It has a very long pelagic or plankton stage, which means that the larvae float freely within the water. This allows for the larvae to spread over large distances. This larva then settles onto the sand or mud and begins to develop into an adult. The body will develop as well as the mantle. The mantle will then secrete and line the shell.

(Kindersley 2001; Ogden 2001)

Genomic DNA is available from 1 specimen with morphological vouchers housed at British Antarctic Survey

US Federal List: no special status

CITES: no special status

Unknown. The Jackknife clam burrows deep and surfaces only to obtain food and water. There does not seem to be any negative effect of the clam on humans.

Ensis directus is a very fast burrower and very difficult to catch while it is still alive. However, when it is caught it can be sold and eaten like many other types of clams. E. directus is in season during the months of July-September.

(Great Northern Products. 2001; Cooper 1960)