The hard clam (Mercenaria mercenaria) or quahog, is an edible marine bivalve mollusc 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 USA are frequently referred to simply as clams, as in 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.
In fishmarkets there are specialist names for different sizes of this species of clam. The smallest 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 ("KWAW-hog", /ˈkoʊhɒg/ "KO-hog", or /kwəˈhɒg/ "kwa-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 the purple color), the species name mercenaria is related to the Latin word for "money."
Many areas where aquaculture is highly important have bred specialized versions of these clams, in an attempt to distinguish themselves in the marketplace. These clams are quite similar, except that their shells bear distinctive markings; for example those from Wellfleet, Massachusetts have pronounced wavy or zigzag lines on their shells, which are meant to be reminiscent to a line of Ws running across the shell — an attempt to capitalize on the town's reputation for oysters. Those that have been specially bred with markings have more recently been breeding with wild stocks, so it is not uncommon for wild clams to have these markings.
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 tiny Rhode Island, situated right in the middle of "quahog country," has supplied a quarter of the USA'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.
In coastal areas of New England, restaurants known as raw 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 (with the toughest meat), are used in such dishes as clam chowder and stuffed clams or are minced and mixed into dishes that use the smaller, more tender clams.
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 fisher 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.
"Quahog" is the name of the fictional Rhode Island city that the hit animated sitcom Family Guy is located in. The city's annual civic celebration is based on the fabled settling of the city, by an immigrant thrown overboard from his ship and saved from drowning by a clam.