|Hard clam or Quahog|
The hard clam (Mercenaria mercenaria), also known as a quahog (or quahaug), round clam, or hard-shell (or hard-shelled) clam, is an edible marine bivalve mollusk which is native to the eastern shores of North America, from Prince Edward Island to the Yucatán Peninsula. It is one of many unrelated edible bivalves which in the United States are frequently referred to simply as clams, as in the expression "clam digging". Older literature sources may use the systematic name Venus mercenaria; this species is indeed in the family Veneridae, the venus clams.
Confusingly, the "ocean quahog" is a different species, Arctica islandica, which, although superficially similar in shape, is in a different family of bivalves: it is rounder than the hard clam, usually has black periostracum, and there is no pallial sinus in the interior of the shell.
Alternative names 
In fish markets there are specialist names for different sizes of this species of clam. The smallest legally harvestable clams are called countnecks, next size up are littlenecks, then topnecks. Above that are the cherrystones, and the largest are called quahogs or chowder clams.
Of all these names, the most distinctive is quahog (pron.: // KWAW-hog, // KOH-hog, or // kwə-HOG). This name comes from the Narragansett word "poquauhock" – the word is similar in Wampanoag and some other Algonquian languages – and is first attested in North American English in 1794. As New England Indians made valuable beads called wampum from the shells, especially those colored purple, the species name mercenaria is related to the Latin word for commerce.
In many areas where aquaculture is important, clam farmers have bred specialized versions of these clams with distinctions needed for them to be distinguished in the marketplace. These are quite similar to common 'wild type' Mercenaria clams, except that their shells bear distinctive markings; for example those from Wellfleet, Massachusetts and elsewhere have pronounced wavy or zigzag chestnut-colored lines on their shells, reminiscent of a line of W's running across the shell. These are known as the notata strain of quahogs, which occur naturally in low numbers wherever quahogs are found.
Hard clams are quite common throughout New England, north into Canada, and all down the Eastern seaboard of the United States to Florida, but are particularly abundant between Cape Cod and New Jersey, where seeding and harvesting them is an important commercial form of aquaculture; for example, the species is an important member of the suspension-feeding, benthic fauna of the lower Chesapeake Bay, while Rhode Island, situated right in the middle of "quahog country," has supplied a quarter of the U.S.'s total annual commercial quahog catch. The quahog is the official shellfish of the U.S. state of Rhode Island. The species has also been introduced and is farmed on the Pacific coast of North America and in Great Britain and continental Europe. It reproduces sexually by females and males shedding gametes into the water.
Quahog Parasite Unknown (QPX) is a parasite that affects Mercenaria mercenaria. While little is known about the disease, research is currently under way in several laboratories. This research is fueled by the need to inform aquaculturists, who suffer financially because of the mortality rates in clams that QPX inflicts and the ensuing years in which runs must be left fallow to clear the disease.
Culinary use 
In coastal areas of New England, New York, and New Jersey, restaurants known as raw bars or clam bars specialize in serving littlenecks and topnecks raw on an opened half-shell, usually with a cocktail sauce with horseradish, and often with lemon. Sometimes, littlenecks are steamed and dipped in butter, though not as commonly as their soft-shelled clam cousin, the "steamer." Littlenecks are often found in-the-shell in sauces, soups, stews, clams casino or substituted for European varieties such as the cockle in southern European seafood dishes. The largest clams, quahogs or chowders and cherrystones, which have the toughest meat, are used in such dishes as clam chowder, clam cakes and stuffed clams, or are minced and mixed into dishes that use the smaller, more tender clams.
Clams and red tide 
The term "red tide" refers to an accumulation of a toxin produced by marine algae. Filter-feeding shellfish — such as clams, oysters, and mussels — are affected. The toxin affects the human central nervous system. Eating contaminated shellfish, raw or cooked, can be fatal. Some other kinds of algal blooms make the seawater appear red, but red tide blooms do not always discolor the water nor are they related to tides.
Clams bought from a market should always be safe, as commercial harvesters are extremely careful about red tides: They close beds that are even remotely threatened and keep them closed for up to three or four weeks after they are clean of any red tide. Commercial clam fishers who are known to break these rules will receive a major fine in the first instance and will most likely have their license to harvest or sell clams revoked; furthermore they are liable for any damages. Clam harvesters who violate the sanitary laws in New York face potential jail terms.
- Harte, M. E. 2001. "Systematics and taxonomy, Chapter 1," pp. 3-51, in Kraeuter, J. N. and M. Castagna (eds.) "Biology of the Hard Clam", Developments in Aquaculture and Fisheries Science, Vol. 31. Elsevier Science B. V. : New York.
- Rice, M.A. (1992). The Northern Quahog: Biology of Mercenaria mercenaria. Rhode Island Sea Grant Publication No. RIU-B-92-001, University of Rhode Island, Narragansett. 60 pp. ISBN 0-938412-33-7.
- "Quahaug, quahog", in Shorter Oxford English Dictionary, 3rd ed (Oxford: Clarendon Press, 1973)
- Roger Williams, A Key Into the Language of America. London: Gregory Dexter, 1643.
- Eldridge, P.J., W. Waltz, and H. Mills. 1975. Relative abundance of Mercenaria mercenaria notata in estuaries of South Carolina. Veliger 18:396-397.
- Calvo, Lisa M. Ragone, Susan E. Ford, John N. Kraeuter, Dale F. Leavitt, Roxanna Smolowitz and Eugene M. Burreson. 2007. Influence of Host Genetic Origin and Geographic Location on QPX Disease in Northern Quahogs (- Hard Clams), Mercenaria mercenaria. Journal of Shellfish Research. 25(1):109-120.
Further reading 
- Kraeuter, J.N. and M. Castagna (eds)., 2001. The Biology of the Hard Clam. Elsevier Science, Amsterdam. 772 pp. ISBN 0-444-81908-8 Google Books
- Perrigault M., Tanguy A. & Allam B. 2009. Identification and expression of differentially expressed genes in the hard clam, Mercenaria mercenaria, in response to quahog parasite unknown (QPX). BMC Genomics 2009, 10:377. doi:10.1186/1471-2164-10-377