The hard clam ( Mercenaria mercenaria), also known as the round clam, hard-shell (or hard-shelled) clam, or the quahog, 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 . 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 a 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.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. web link .
The most distinctive of these names is quahog ( , , or , also spelt quahaug, quohog or cohog).
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.Eldridge, P.J., W. Waltz, and H. Mills. 1975. Relative abundance of Mercenaria mercenaria notata in estuaries of South Carolina. Veliger 18:396-397.
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 and is the namesake of the fictional Rhode Island town featured in the animated sitcom Family Guy. 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 X (or quahog parasite unknown QPX) disease of the hard clam Mercenaria mercenaria is caused by a poorly known protistan parasite. Its DNA sequence analysis places the QPX parasite among the thraustochytrid . The QPX parasite is unicellular and possesses at least one flagellum, like most other stramenopile organisms. Thraustochytrids are common protists in marine sediments and the water column, but only a few thraustochytrids are known as parasites of marine animals. Although QPX disease was first recorded on the Atlantic coast of Canada in the early 1960s, it did not become a major economic problem until its appearance in cultured clams at Prince Edward Island, Massachusetts in 1992, and Virginia in 1997. Infected clams are characterized by the presence of blisters or pustules in the mantle and later by gaping and death.
Historically, Native Americans used the quahog as a component in wampum, the shell beads exchanged in the North American fur trade. The Narragansetts used the hard clam for food and ornaments.
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.
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