A littleneck clam is one of two clam species, both referred to as littleneck. Protothaca stamineals, also called the Pacific or common littleneck, is found along the Pacific coast of North America. Mercenaria mercenaria, commonly called the quahog clam, is native to the Atlantic coast and the Gulf of Mexico. In this case the term “littleneck clam” refers to a particular size of clam, usually in the range of 2 to 2.5 inches (5.1 to 6.3 cm) measured across the longest part of the shell. Although both species are harvested commercially, the quahog clam is far more important economically.
Pacific littleneck clams are small, reaching a maximum of 2.5 inches (6.3 cm) across. They have oval shells marked by both prominent ribs that radiate from the hinge to the edges and faint concentric circles. A heart-shaped depression in the shell can be distinguished near the hinge knob.
Shell colors vary depending on habitat. Clams found in estuaries and bays often have gray to grayish-yellow exterior shells, whereas those from more open waters tend to be white with brown patterns or blotches. Shell shape also varies, and individuals may be broader and more compact, or stretched longer with a thinner profile.
These clams live in both sand and mud areas in the lower intertidal zone. They are filter feeders and live on microscopic plants and animals drifting in the currents. In turn the clams provide food for many animals including seabirds, some types of fish, sea otters and rock crabs. Commercial harvesting of the Pacific littleneck clam began in the late 20th century and they form a growing part of the Pacific coast shellfish industry.
Mercenaria mercenaria is the other littleneck clam. The species as a whole is called the quahog, or sometimes Northern quahog, clam; an individual ranging in size from about 2 to 2.5 inches (5.1 to 6.3 cm) is called a littleneck clam. This species is native from the Gulf coast of Texas east to Florida and up the Atlantic coast to the Gulf of St. Lawrence in Canada.
These clams reach a maximum size of 5 inches (about 13 cm) across. The shells of younger individuals are covered by concentric circular ridges radiating from the hinge knob. These wear off over time so that adult shells may be mostly smooth. Their exterior shells are off-white, light brown, gray or brownish gray. Like their Pacific counterparts, these clams are filter feeders found on both sandy and muddy areas in the intertidal zone.
Quahog clams are among the most important commercial shellfish species in the Atlantic. They are not only harvested along the North American coast but also have been successfully transplanted to the waters off of Great Britain, and those areas serve a growing European market. Attempts have been made to introduce them along the European Atlantic and Mediterranean coasts, but none of these populations are considered well established.