Mercenaria mercenaria (Linnaeus, 1758)

Common name(s): Northern quahog

Synonyms:  Venus mercenaria Mercenaria mercenaria
Phylum Mollusca
 Class Bivalvia
  Subclass Heterodonta
   Order Veneroida
    Family Veneridae
Mercenaria mercenaria from a local market.
(Photo by: Dave Cowles, July 2007)
Description:  As with other clams of family Veneridae (a very large family called the Venus clams), this species has a porcelain-type (chalky) shell, the umbo is anterior to the midline (and to the hinge) but still closer to the center of the shell than to the anterior end, the hinge has 3 cardinal teeth in each valve (photo), and a definite pallial sinus is present (photo).  There are no anterior lateral teeth.  Other characteristics of family Veneridae include: 2 valves more or less oval or heart shaped, similar in size, and at least 1/4 as wide as long, valves are not subdivided into sections by major grooves or differences in texture, do not have winglike extensions near the umbo, have two adductor muscle scars of similar size (photo), have no chondrophore, pallial line is continuous (photo), hinge ligament is mostly or completely external (photo).  This species may have radial ribs but they are not as prominent as those in our native littleneck clam, Protothaca staminea.  The hinge ligament is about 1/3 the length of the shell (photo).   The valves have a row of very small teeth along the inside of the ventral margin (photo).  The pallial sinus is deep and pointed (photo).  The shell is about as high as it is long and is usually whitish or gray on the outside and may or may not have a little yellow-brown periostracum.  The concentric growth lines on the external shell are fairly smooth.  The shell has a deep lunule and distinct beaks at the umbo.

How to Distinguish from Similar Species:  This clam, which is not native to the Northwest, can look quite similar to our native littleneck clam Leukoma stamineaL. staminea is of similar size and shape and also has the distinctive fine teeth along the inside ventral margin of the valves.  However, from my observations L. staminea usually has more abundant and prominent radial ribs than does Mercenaria mercenaria, and it may also have colorful markings on its periostracum which M. mercenaria usually lacks.

Geographical Range:  Atlantic Ocean (Gulf of Saint Lawrence to Florida, Texas), introduced in some areas of our Pacific coast, especially in San Francisco Bay and several other areas of California, as well as a small population in southern British Columbia.

Depth Range:

Habitat:  In stable sand, packed mud, or gravel-clay mixtures in protected areas, usually buried less than 8 cm below the sediment.

Biology/Natural History:  This species is a very important commercial species from the east coast, which has been successfully established in several areas of California and also in southern British Columbia.  Spawns in 24-25 C water.  Anticarcinogenic agents have been isolated from the tissue.

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Dichotomous Keys:
  Carlton, 2007
  Smith and Carlton, 1975

General References:
  Morris et al., 1980
  Ricketts et al., 1985

Scientific Articles:

Web sites:

General Notes and Observations:  Locations, abundances, unusual behaviors:

With radial ridges

Some individuals have radial ridges on their shell, as seen in this specimen.  The lunule cannot be clearly seen but it is in the flattened depression on the top right.

Hinge--inside view

The hinge teeth are prominent.

In this inside view of the shell can be seen the prominent, continuous pallial line with a distinct, deep, pointed pallial sinus.  Note also that there are two similarly-sized adductor muscle scars and the hinge ligament is external and about 1/3 the length of the shell.  The anterior end of the shell is down in this view.

This closeup of the hinge shows the three large cardinal teeth in each valve which is characteristic of Family Veneridae
Anterior is down in this view.  The left valve is to the left and the right valve is to the right.

Authors and Editors of Page:
Dave Cowles (2008):  Created original page