Mercenaria mercenarianorthern quahog

Geographic Range

Mercenaria mercenaria live in intertidal zones at depths of up to 10 m (Britannica 2000). This species' native distribution is along the east coast of North America, from the Gulf of St. Lawrence into the Gulf of Mexico (Mercenaria mercenaria 2000, Plourde 2001). However, it has been introduced to other areas including the coasts of California, England, Humboldt Bay, and Southern Brittany (Mercenaria mercenaria 2000, Mitchell 2001). Attempts were also made to introduce the clam to the Etang de Thau on the south coast of France and around Sicily, but no populations have successfully established in the Mediterranean Sea (Mercenaria mercenaria 2000). There have been other attempts to introduce the clams but most don't result in a self-sustaining population (Mitchell 2001).


Mercenaria mercenaria are found in the intertidal zone from the Gulf of St. Lawrence to the Gulf of Mexico (Britannica 2000). They are found most abundantly on mud flats, mud/sand flats, and sand flats which reach depths of 10 m (Plourde 2001, Stewart 1996). However, soft muddy bottoms cannot support the weight of the clam due to its heavy shell. This causes sediments to be stirred up, and silt may block the siphon of the clam from filtering out the water (Stewart 1996). Also, it is important that they have tides in order to receive food and oxygen and to carry away waste, but turbulent waters, found in the surf zone, may wash them away (Stewart 1996). The perfect salinity range for Mercenaria mercenaria larvae are 20 to 35 parts per thousand, however adults can be more tolerant (Stewart 1996). The Chesapeake bay is an excellent habitat because it provides the needed salinity and temperatures that are optimal for the hard-clam's survival.

Physical Description

Mercenaria mercenaria has a fairly large and thick shell with uneven, elevated hinges on the anterior (Mercenaria mercenaria 2000). Both shell halves are approximately even in size and sub ovate, or triangular in shape (Stewart 1996). The shell is increased by a daily layering of aragonite secreted by the outermost fold of the organism, giving it numerous concentric lines that are closely space near the margins of the shell and widely spaced at the umboes (deMenocal 2000, Mercenaria mercenaria 2000). Its shell is a composition of proteins and calcium carbonate (Plourde 2001, Brown 1995). The shell is joined at a hinge called the umbo and is held closed by two pairs of adductor muscles located on each side of the shell (Plourde 2001). The clam opens its shell by relaxing the adductor muscles and contracting a pair of ligaments located on each side of the umbo (Plourde 2001). Mercenaria mercenaria contain three well-developed teeth located on the edge of the shell, which serve to enhance its tightness when closed (Stewart 1996). The external color is dirty white or gray, while the interior is usually white with distinct violet areas near the umbo. The clam contains a foot, which allows it to burrow into the sand (Plourde 2001). The clam also contains a set of long siphons, which stretch from the clam's mantle, the membranous sac that contains the internal organs and constitutes the body of the clam, to the surrounding medium outside of the shell (Plourde 2001). It uses these siphons for respiration and gathering food. Mercenaria mercenaria are sub classified by length. Chowders are the biggest measuring up to 3 inches in width, Cherrystones are 2 to 3 inches, Top Necks are 2 inches in width, and Little Necks are the smallest measuring 1 to 2 1/3 inches (International Seafood).


When the water temperatures approach 23°C (73°F), and other environmental cues such as pH change, spawning commences(Britannica 2000, Stewart 1996). Some species have adapted to spawn at temperatures ranging from 4°C and lower (Mitchell 2001). The male clam discharges sperm into the water, which stimulates the female to release eggs (Stewart 1996). Because fertilization is random, high densities of spawning clams increase the probability of success (Cool 1998). Due to favorable conditions, spawning is most likely to occur during neap tides (Stewart 1996). During the first 12 to 14 hours, the fertilized eggs turn into trochophore larvae (Stewart 1996). In this form, they are cylindrical with tiny cilia, which allow them to swim about (Stewart 1996). During this period they feed on diatoms, microscopic algae that are encased in silica shells (Stewart 1996). By the end of the first day, the trochophore larvae transform into veliger larvae, which contain tiny lobes that may be used as paddles (Stewart 1996). During the next six to ten days, the body organs, shell and foot begin to form (Stewart 1996). They then shed their lobes and the newly developed foot secretes byssal threads that anchor the larvae to rocks, seaweed, or other sediment deposits (Plourde 2001, Stewart 1996). They will then secrete their shell, which begins to calcify between 8- 29°C, and detach the byssal threads, thus becoming adult clams (deMenocal 2000). During their free floating period they are dispersed by currents and preyed upon by other animals such as crabs. Approximately 10% of all veliger larvae reach the adult stage (Stewart 1996). Temperatures that vary below 80°C may reduce the growth stages dramatically (Mercenaria mercenaria 2000).


Due to the fact that Mercenaria mercenaria spend their lives in an immobile and isolated state, their behavior isn't that extensive. In case of danger they are capable of retracting their siphons and some clams have been known to migrate small distances. They are capable of burrowing deep under the sediment layers using their foot structure.

In clams, there are signs of chemotaxis, movement toward a chemical gradient, of hemocytes, phagocytic leukocytes, to sites of injury (Fawcett 1994). This is similar to the primary immune response of inflammation due to bacterial infection in vertebrates. These inflammatory responses can be induced by injections of such harmful bacteria as Escherichia coli which release a peptide, attracting the hemocytes (Fawcett 1994).

The autorhythmicity, or spontaneous beating of the ventricle, in Mercenaria mercenaria, is dependent on the extracellular levels of potassium (Devlin 1993). By regulating the amount of potassium ions, the fast rising rate, determined by Ca2+, and systolic beating, determined by Na+, do not reach a plateau phase (Devlin 1993).

Mercenaria mercenaria are preyed upon by such organisms as oyster drills, moon snails, whelks, drums, skates, pufferfish, rays, and certain water fowl (Stewart 1996). Certain parasites, such as the polychaete worm, cause blisters to form inside the shell, which may cause death indirectly (Stewart 1996).

Food Habits

Mercenaria mercenaria are suspension feeders which means that they feed on small plants and animals called plankton which are drawn in with water (Britannica 2000, Plourde 2001). When the clam buries itself under a layer of silt and mud it sticks its siphons straight up through the surrounding muck. The inhalant siphon draws in water, which is passed over the gills. Millions of tiny cilia, hair-like structures, move the water across the gills and any food particles are caught in a mucous sheet that coats the gills (Plourde 2001, Stewart 1996). This food-mucous mixture is passed along a groove above the foot to a pair of muscles called the palps, which force the material into the mouth (Plourde 2001). It then follows the digestive tract consisting of a stomach, intestine, and anus to be excreted through the exhalent or excurrent siphons, as pseudofeces (Britannica 2000, Plourde 2001).

Economic Importance for Humans: Positive

Mercenaria mercenaria is an edible species, and is the most important commercially harvested species in the Virginia Bay, raking in approximately $4 million to $6 million annually (Cool 1998). In addition, they also filter and recycle organic material in the Chesapeake Bay, removing toxic materials and clarifying the water (Cool 1998). With the introduction into Great Britain in 1960 they have become a large economic market in France and Italy where introduction attempts have already failed (Stewart 1996). Also, it is thought that the liver may contain chemicals that have selectivity for cancerous cells (Mercenaria mercenaria 2000).

Economic Importance for Humans: Negative

In Britain, a cold spell during 1947, 1962, and 1963 killed off the entire soft-shelled clam, Mya arenaria, population (Mitchell 2001). With the introduction of Mercenaria mercenaria the niche was filled and, and the Mya arenaria population has never recovered (Mitchell 2001).

Conservation Status

The Virginia Marine Resources Commission has designated hard-clam broodstock sanctuaries in order to increase the probability of fertilization thus increasing the population (Cool 1998). The Hard Clam Broodstock Program piloted a site at Lower Brown Shoals in April 1995 for hard-clam sanctuaries (Cool 1998). The Back River Reef BroodStock Sanctuary, created February1997, has kept the commercial fishing and natural predators from preying on the hard-clams in that area (Cool 1998). And, the Middle Ground Light Broodstock Sanctuary, created March 28, provides good disbursement for larvae (Cool 1998). These are all attempts to increase the amount of commercial supplies while maintaining the clam population.

Other Comments

The clam's history with introduction into England has shown that deliberate introduction of a species has the capability of working commercially (Mercenaria mercenaria 2000). The Algonquin tribes would grind up the shells and make beaded necklaces that would be used as wampumpeag or wampum, which means money (Grzimek's 1974). It is thought that quahog is the corruption of "Poquahock" which is the Narragansett Indian name for Mercenaria mercenaria (Plourde 2001). When European settlers came to America they used the word "clamps" because of the locking system the animals had, which was eventually shortened to "clams" (Plourde 2001).


Bradford Burdette (author), Western Maryland College, Louise a. Paquin (editor), Western Maryland College.


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.

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Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

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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.


the nearshore aquatic habitats near a coast, or shoreline.


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


referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.

native range

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


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Encyclopedia Britannica Online, 1994-2001. "clam" (On-line). Accessed May 1, 2001 at

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Owen, G. 1966. Feeding. Pp. 1-52 in K Wilbur, C Yonge, eds. Physiology of Mollusca: Volume II. New York: Academic Press Inc..

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deMenocal, P. December 3, 2000. "The Paleotemperature Archive: Mercenaria mercenaria -- the common quahog" (On-line). Accessed March 19, 2001 at