Clam is a common name for several kinds of bivalve molluscs. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two shells of equal size connected by two adductor muscles and have a powerful burrowing foot. They live in both freshwater and marine environments; in salt water they prefer to burrow down into the mud and the turbidity of the water required varies with species and location; the greatest diversity of these is in North America.
A clam's shell consists of two (usually equal) valves, which are connected by a hinge joint and a ligament that can be external or internal. The ligament provides tension to bring the valves apart, while one or two adductor muscles can contract to close the valves. Clams also have kidneys, a heart, a mouth, a stomach, and a nervous system. Many have a siphon.
In culinary use, within the eastern coast of the United States and large swathes of the Maritimes of Canada, the term "clam" most often refers to the hard clam Mercenaria mercenaria. It may also refer to a few other common edible species, such as the soft-shell clam, Mya arenaria and the ocean quahog, Arctica islandica. Another species commercially exploited on the Atlantic Coast of the United States is the surf clam Spisula solidissima. Scallops are also used for food nationwide, but not cockles: they are more difficult to get than in Europe because of their habit of being farther out in the tide than European species on the West Coast, and on the East Coast they are often found in salt marshes and mudflats where mosquitoes are abundant. Up and down the coast of the Eastern U.S., the bamboo clam, ensis directus, is prized by Americans for making clam strips although because of its nature of burrowing into the sand very close to the beach, it cannot be harvested by mechanical means without damaging the beaches. The bamboo clam is also notorious for having a very sharp edge of its shell, and when harvested by hand must be handled with great care.
In Kerala clams are used to make curries and fried with coconut. In Malabar region it is known as "elambakka" and in middle kerala it is known as "kakka". Clam curry made with coconut is a dish from Malabar especially in the Thalassery region. On the south western coast of India, also known as the Konkan region of Maharashtra, clams are used in curries and side dishes, like Tisaryachi Ekshipi, which is clams with one shell on. Beary Muslim households in the Mangalore region prepare a main dish with clams called Kowldo Pinde. In Udupi and Mangalore regions it is famously called as "marvai" in local tulu language. It is used to prepare many delicious dishes like marvai sukka, marvai gassi, and marvai pundi.
The rocky terrain and pebbly shores of the seacoast that surrounds the entire island provides ample habitat for shellfish, and clams are most definitely included in that description. The oddity here is that for a nation whose fortunes have been tied to the sea for hundreds of years, 70% of the seafood cultivated for aquaculture or commercial harvesting is exported to the Continent. Historically, Britain has been an island most famous of all for its passion for beef and dairy products, although there is evidence going back to before most recorded history of coastal shell middens near Weymouth and present day York. (There is also evidence of more thriving local trade in sea products in general by noting the Worshipful Company of Fishmongers was founded in 1272 in London.) Present day younger populations are eating more of the catch than a generation ago, and there is a prevalence of YouTube videos of locavore scavenging, however the numbers have a long way to go before they match the numbers consumed in Mesolithic, as evidenced by the strikingly large number of shells found in said middens.
In Italy, clams are often an ingredient of mixed seafood dishes or are eaten together with pasta. The more commonly used varieties of clams in Italian cooking are the Vongola (Venerupis decussata), the Cozza (Mytilus galloprovincialis) and the Tellina (Donax trunculus). Though Dattero di mare (Lithophaga lithophaga) was once eaten, overfishing drove it to the verge of extinction (it takes 15 to 35 years to reach adult size and could only be harvested by smashing the calcarean rocks that form its habitat) and the Italian government has declared it an endangered species since 1998 and its harvest and sale are forbidden.