Mercenaria mercenaria has a thick shell, roughly triangular in shape overall, light brown to grey in colour with a violet border and often with varying concentric bands on the shell . These concentric bands are conspicuous and are closely spaced around the margins but more widely spaced around the umbo. The inner shell surface is shiny with a purplish-blue tinge around the muscle scars. The sculpture of the shell consists of thin concentric ridges that are sharp and raised in early growth stages but worn away in older shells. It can grow up to 12 cm in length. The beak extends well beyond the the main shell. The pallial line is short and triangular with a finely rippled inner margin. Each valve has three conspicuous teeth. The internal anatomy is distinctive. The exhalent and inhalent siphons are joined with a fringe of tentacles around the inhalent siphon. The siphons are yellowish or brownish orange at the ends and often streaked with dark brown or opaque white. The foot is large and white in colour.Commonly known as the Quahog and used to make clam juice. This is a non-native species unsuccessfully introduced into British waters several times since the middle of the nineteenth century. The first live specimen was found in 1864 in the Humber. Merceneria mercenaria was successfully introduced from the USA into Southampton Water in 1925. Due to the colouration of the shell, the native American Indians utilised the shell as 'wampun' for use as currency, hence its scientific name.

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).

Biogeographic Regions: atlantic ocean (Native ); pacific ocean (Introduced )

Newfoundland to Florida and Texas

Atlantic Europe, Belgian Exclusive Economic Zone, Birch Cove, East Coast of England, European waters (ERMS scope), Gulf of Maine, Gulf of Mexico, Irish Exclusive economic Zone, Mediterranean Sea, North Adriatic, North Sea, North West Atlantic, Sam Orr Pond, South Coast of England, United Kingdom Exclusive Economic Zone, Wimereux

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

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).

The overall shape of the shell is trigonal-ovate. The margin is rounded anteriorly, and lightly pointed posteriorly. The valves are the same size and shape (equivalve), and there is no gape between them when closed. Ornamentation consists of growth lines and comarginal ribs. The ribs are fine and closely spaced. At the anterior and posterior they become more concentrated and stronger, while the central area is smoother. The lunule is impressed and rounded so the length and width are similar in measure; it extends half of the anterior dorsal margin. The umbo points anteriorly and projects dorsally when the valves are viewed from the anterior. The escutcheon is lightly impressed, but otherwise not well defined.

The exterior color varies from a light tan to a light grey. There can also be blotches of brown. The interior color is white, sometimes with purple coloration from the pallial line to the margin and in the pallial sinus where the color becomes more concentrated.

The ligament extends exteriorly and its length is approximately half of the posterior dorsal margin. The ligament is supported by nymphal ridges. There are three cardinal teeth on each valve. On the left valve, the anterior tooth has a shallow groove, the central tooth is bifid, and the posterior tooth is fused with the nymphal ridge and is slightly rugose. Lateral teeth are absent. On the right valve, the anterior tooth is simple, and the central and posterior teeth are bifid. There are crenulations on the interior margin, extending throughout the anterior dorsal to the posterior end of the ventral margin. The anterior and posterior adductor muscle scars are similar in shape and size. The pallial sinus is tapering and it almost extends to the posterior end of the ligament in length.

Original description, by Linnaeus (1758, p. 686):
"V. testa cordata solida transverse substriata laevi, margine crenulato, intus violacea, ano ovato.
List. angl. 229. t. 8. f. 33.
Habitat in Pensylvania. P. Kalm: e qua Sylvestrium Nummi parantur. In montibus Sveciae fossilis.
Testa prae reliquis crassa est & ponderosa. Limbus tantum testae interne violacens est."

Similar to Mercenaria campechiensis. Mercenaria mercenaria is distinguishable by a central smooth region on the exterior of the valves, with stronger ribs anteriorly and posteriorly; whereas, Mercenaria campechiensis can be distinguished by ribs that become more dense and fine as the animal grows.

Similar to Pitar morrhuanus. Mercenaria mercenaria can be distinguished by its lack of lateral teeth and the presence of a central smooth region on the exterior of its valves.

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.

Aquatic Biomes: coastal

intertidal and infralittoral of the Gulf and estuary

Depth range based on 429 specimens in 2 taxa.
Water temperature and chemistry ranges based on 10 samples.

Environmental ranges
  Depth range (m): 0 - 104.8
  Temperature range (°C): 9.208 - 24.381
  Nitrate (umol/L): 0.457 - 3.829
  Salinity (PPS): 32.282 - 35.844
  Oxygen (ml/l): 4.845 - 6.764
  Phosphate (umol/l): 0.110 - 0.547
  Silicate (umol/l): 0.868 - 2.666

Graphical representation

Depth range (m): 0 - 104.8

Temperature range (°C): 9.208 - 24.381

Nitrate (umol/L): 0.457 - 3.829

Salinity (PPS): 32.282 - 35.844

Oxygen (ml/l): 4.845 - 6.764

Phosphate (umol/l): 0.110 - 0.547

Silicate (umol/l): 0.868 - 2.666
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

Habitat Type: Marine

 Found buried in muddy sediment on the lower shore and shallow sublittoral and in bays and estuaries. Prefers sandy environments to depths of 15 m.

The natural range of distribution of the northern quahog Mercenaria mercenaria spreads along the east coast of the United States, but the species was intentionally introduced in France in 1861 for farming purpose. The northern quahog was later introduced into several other European countries with the same goal. Later on, the species was often transported by accident together with oysters. The first Belgian specimens were found in the Spuikom in Ostend around 1933. In the 1990’s and the start of this century, the species was reintroduced several times for farming purposes.

Non-Migrant: No. All populations of this species make significant seasonal migrations.

Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).

Locally Migrant: No. No populations of this species make annual migrations of over 200 km.

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).

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).

Genomic DNA is available from 1 specimen with morphological vouchers housed at Australian Museum, Sydney

The following is a representative barcode sequence, the centroid of all available sequences for this species. 

 
There are 93 barcode sequences available from BOLD and GenBank.  Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species.  See the BOLD taxonomy browser for more complete information about this specimen and other sequences.
 

Download FASTA File

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 93
Species: 102
Species With Barcodes: 1

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.

US Federal List: no special status

CITES: no special status

United States

Rounded National Status Rank: N5 - Secure

Rounded Global Status Rank: G5 - Secure

Reasons: Common marine species (Abbott, 1968).

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).

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).