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 that is native to the eastern shores of North America and Central America from Prince Edward Island to the Yucatán Peninsula. It is one of many unrelated edible bivalves that 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 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.
In fish markets, there are specialist names for different sizes of this species of clam. The smallest legally harvestable clams are called countnecks or peanuts, next size up are littlenecks, then topnecks. Above that are the cherrystones, and the largest are called quahogs or chowder clams.
The most distinctive of these names is quahog (// KOH-hog, // KWAW-hog, or // kwə-HOG). The word comes from the Narragansett word "poquauhock", which is similar in Wampanoag and some other Algonquian languages; it is first attested in North American English in 1794. New England tribes made valuable beads called wampum from the shells, especially those colored purple; the species name mercenaria is related to the Latin word for commerce. Today people living in coastal New England still use the Native American word for the clam as they have done for hundreds of years.
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. 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 they 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.
Rhode Island is situated right in the middle of "quahog country" and 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. It was discovered along the coast of Cape Cod, Massachusetts in 1995.
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, and clams casino, or substituted for European varieties such as the cockle in southern European seafood dishes. The largest clams are quahogs or chowders and cherrystones; they have the toughest meat and 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.
A population of hard clams exists in Southampton Water in Hampshire, England. Originally bred in the warm water outflows at Southampton Power Station for use as eel bait, the population became self-sustaining and can now be found in Southampton Water and has also spread to Portsmouth Harbour and Langstone Harbour.
Clams and red tide
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The term "red tide" refers to an accumulation of a toxin produced by marine algae. Filter-feeding shellfish are affected, such as clams, oysters, and mussels. 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.
Clam harvesters who violate the sanitary laws in New York face potential jail terms.
Mercenaria mercenaria reproductive biology
The reproductive biology of the hard clam Mercenaria mercenaria has been described in detail in the book “Biology of the Hard Clam” in 2001. The first published report on the electron microscope study of the spermatozoa of the hard clam Mercenaria mercenaria in 1972 identified its acrosome is involved in the hard clam’s external mode of fertilization by extrusion of the acrosome filament which occurs during the process of sperm penetration through the egg investment in the mollusk. The above finding has been subsequently confirmed in 2008 by a comprehensive study on the ultrastructure of the spermatozoa of Mercenaria mercenaria.
- 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 web link.
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- Calvo, LMR; Ford, S. E.; Kraeuter, J. N.; Leavitt, D. F.; Smolowitz, R.; Burreson, E. M. (1 January 2007). "Influence Of Host Genetic Origin And Geographic Location On Qpx Disease In Northern Quahogs (=Hard Clams), Mercenaria Mercenaria". Journal of Shellfish Research. 26: 109–119. doi:10.2983/0730-8000(2007)26[109:IOHGOA]2.0.CO;2.
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- Feeney, Kathy (2003). Rhode Island Facts and Symbols. Albert T. Klyberg (consultant) (Revised and Updated ed.). Mankato, Minn.: Capstone Press. pp. 18–19. ISBN 9780736822701. OCLC 51204649.
10. Eversole, A. G. 2001. "Reproduction in Mercenaria mercenaria, Chapter 5", pp. 221-–260, 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. doi:10.1016/s0167-9309(01)80033-8.
11. Lin, Chi-Chung (1972). "Electron microscope study of the spermatozoa of the hard-shelled clam Venus mercenaria (Mollusca) by negative staining". Transactions of Kentucky Academy of Science, 33: 57-63.
12. Ying, Xue-Ping; Yang, Wan-Xi; Dahms, Hans-U; Lin, Zhihua; Chai, Xueliang (2008) "Spermatozoa and spermatogenesis in the northern quahaug Mercenaria mercenaria (Mollusca, Bivalvia)". Helgoland Marine Research, 62: 321-329.
- 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