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Ensis directus

By Kimberly Camponelli

Geographic Range

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)

Habitat

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)

Physical Description

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)

Reproduction

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)

Behavior

Ensis directus burrows into the sand using its foot and it only surfaces during high tide. It will climb close to the surface so that only its siphons are exposed. These two siphons are used for filtering food and water. When low tide occurs, it burrows back down below the surface. E. directus is an extremly fast burrower and is very hard to be caught due to its great speed. It is also a remarkable swimmer. It is able to propel itself through water by expelling water through its shell and drawing in its foot. This action is repeated over and over again allowing it to move through the water.

(Alexander 1979; Cooper 1960)

Food Habits

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)

Economic Importance for Humans: Positive

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)

Economic Importance for Humans: Negative

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.

Conservation Status

Contributors

Kimberly Camponelli (author), Western Maryland College, Louise a. Paquin (editor), Western Maryland College.

Glossary

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

coastal

the nearshore aquatic habitats near a coast, or shoreline.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

native range

the area in which the animal is naturally found, the region in which it is endemic.

References

Alexander, R. 1979. The Invertebrates. Great Britain: Cambridge University Press.

Cooper, E. 1960. Science on the Shores and Banks. New York: Harcourt, Brace and Company.

Gosner, K. 1978. Atlantic Seashore. New York: Houghton Mifflin Company.

Great Northern Products, 2001. "Molluscs and Crustaceous" (On-line). Accessed April 27, 2001 at http://northernproducts.com/html/molluscs___crustaceous.html.

Jobin, A., R. Jobin. 1997. "The Assateague Naturalist" (On-line). Accessed April 20, 2001 at http://www.assateague.com.

Kindersley, D. "Mollusca - Anatomy" (On-line). Accessed May 9, 2001 at http://www.chardikalaa.com/~gurtej/hmwk/bio/mollusca/.

Lippson, J. 1984. Life in the Chesapeake Bay. Baltimore: Johns Hopkins University Press.

Ogden, M. "Mollusca" (On-line). Accessed May 9, 2001 at http://www.personal.psu.edu/users/m/a/mao153/.

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