Clams and mussels are very similar in appearance, but they are actually two different types of shellfish. Shellfish come in five different types, and those five varieties can have their own subcategories. These shellfish have distinct characteristics and are often specific to one part of the world. They are all edible seafood, although their preparation and best method of cooking can vary.
Hard-shell clams, called quahogs, are scattered throughout the United States' Eastern coast. They come in three main sizes: chowders are the largest at about 3 inches in diameter; cherrystones are around 2 inches; and little necks are the smallest at about an inch. These clams keep their shells closed tightly; you won't have to wash sand out of the inside to clean them. Do not cook a quahog whose shell is easily opened; this means the clam has gone rotten.
Soft-shell clams are called steamers and are found in muddy coastal areas like the waters of the Chesapeake Bay and elsewhere in New England. The clam meat is still fresh if you poke the clam and it retracts farther into the shell. Thoroughly wash or soak soft-shell clams to get rid of all the sand that accumulates inside them; rinse them in running cold water and soak them overnight in a mixture of salt water and cornmeal. Soft clams are often served fried, steamed -- though that is not where they got their name from -- or eaten raw on the half shell.
Mussels have a noticeably longer and thinner shape than clams. Zebra mussels, named for the striped pattern on their shells, are found in fresh water, including many rivers and lakes. Adult mussels can be found attached to rocks, boats and other objects. Blue mussels have dark-coloured shells and are often found up north in polar waters; they usually grow to 3 to 4 inches in length. Horse mussels are the largest type at around 7 inches and are also stationary shellfish. Other mussels like snuffbox and rabbitsfoot are rare and declared endangered species.
Oysters are found along both American coasts: the Viriginica oysters in the East and the Pacific oysters in the West. The Pacifics were imported from Japan to replace the endangered Olympias. Shrimp is one of the largest types of shellfish that is farmed for food; wild types like cleaner and pistol shrimp are much smaller. Scallops are divided into three groups: sea scallops, which are the largest and found around the northeast, Bay scallops that are found all along the Atlantic coast and Calico scallops in the Gulf of Mexico.