Shellfish are separated into two categories: crustaceans and mollusks. Crustaceans have elongated bodies and external shells with jointed parts. Crab, lobster and shrimp are examples of crustaceans. Mollusks have soft, tender bodies that are in most cases covered by a shell. Mollusks are classified as univalves, bivalves and cephalopods. Univalves, or single-shelled mollusks, include abalone, sea urchins and conch. Bivalves, or double-shelled mollusks, include clams, oysters and mussels. Cephalopods have ink sacs and tentacles. Octopus and squid are two examples of cephalopods.

Clams are found along both the Atlantic and Pacific coasts of the United States. Clams may have a hard or a soft shell. Atlantic soft-shell clams include steamers and longneck clams. Atlantic hard-shell clams include the small littleneck, the medium cherrystone and the large chowder clams. All but the chowder clams may be eaten raw. Pacific hard-shell clams include littleneck and butter varieties. Pacific soft-shell clams are the razor and geoduck clams. Pacific clams are cooked, since they are generally too tough to eat raw.

Several forms of crabs are available in the United States. Dungeness crabs from the Pacific Northwest and blue crabs from the Atlantic and Gulf of Mexico coasts have edible bodies and claws. Alaska king crabs from the northern Pacific have edible meat in the claws and legs. Stone crabs, which come from the waters around Florida, have edible meat only in the claws. Canned crabmeat is usually taken from snow crabs, although sometimes it is taken from Alaska king or Dungeness crabs.

Two forms of lobsters are found in the waters surrounding the United States. The most common is the American or Maine lobster from New England. The meat, which is contained in the tail and the claws, is sweet and tender. Spiny lobsters, which are also known as rock lobsters, come from California and the Gulf of Mexico. They have rough reddish-brown shells and no claws. The meat comes from the tail.

Mussels can be found in the waters of both the Pacific and Atlantic coasts. However, most mussels found in markets are farm raised. They have bluish-black shells and are usually sold in the shell.

Oysters are found on the Atlantic, Pacific and Gulf of Mexico coasts and in Puget Sound in the Pacific Northwest. The oysters from the Pacific coast are quite large (up to a foot long) and those from Puget Sound are small (1-1/2 inches). Pacific oysters are cut up and cooked, whereas those from Puget Sound are eaten raw. Atlantic oysters, or Eastern oysters, better known by their place of origin (such as Blue Point, Chesapeake and Indian River), are the most common oysters. They range in size from 3 to 5 inches in length. They are eaten raw on the half shell or cooked. Many of these varieties are now farm raised.

Shrimp are found in most of the waters surrounding the United States. Shrimp are grouped for retail purposes by their size. The most common sizes are jumbo (11 to 15 per pound), large (21 to 30), medium (31 to 35) and small (36 to 45).

All fresh shellfish should have a mild aroma and smell of the sea. Avoid shellfish that have a strong fishy odor. Fresh shellfish are very perishable. To find the freshest shellfish available, buy from a reputable fish market or supermarket that has a rapid turnover. If you are in doubt about the freshness of any shellfish, do not buy it. If you are purchasing live shellfish, it is best to buy them as close to the time you plan to cook them as possible. Fresh shellfish should be cooked on the day they are purchased. Frozen shellfish should be packaged in a close-fitting moisture-proof package that is intact at the time of purchase.

Live lobsters and crabs should feel heavy for their size and actively move their claws. Lobsters should flap their tail tightly against their chests or curl their tail under their shell when picked up. However, if the lobsters and crabs have been refrigerated, they will not be as active. Do not purchase any lobsters or crabs that do not show these signs of life. Frozen lobster tails should be intact in packaging and free of ice crystals and juices. Whole crabs and crabmeat should smell sweet, not fishy. Lump crabmeat is the large pieces of white crabmeat usually used for salads. Backfin crabmeat is the smaller pieces that are used when appearance is less important (for example, crab cakes).

Shrimp are available raw or cooked, fresh or frozen, and unshelled or peeled. All should feel firm to the touch. Cooked shelled shrimp should be plump. Raw shrimp should not smell of ammonia.

Clams, mussels and oysters should have moist shells free of cracks and chips. All hard-shell clams, mussels and oysters should have tightly closed shells or slightly open shells that snap tightly closed when tapped. If they do not close when tapped, they are dead and should be discarded. The soft-shell clam is unable to close its shell completely. To determine if it is alive, gently touch the protruding neck of the clam to see if it will retract. If the neck does not retract slightly, discard the clam. Freshly shucked clams are packaged in their liquor and should be plump and moist. Their color varies from grayish-green to beige to dark orange depending on the variety. Shucked mussels should be plump and their liquor clear. Freshly shucked oysters should be surrounded by a clear, slightly milky white or light gray liquid. Oysters are usually creamy white but the color varies depending on the variety. Freshly shucked scallops vary in color from creamy white to pink.

How Much to Buy (per person):

Lobster (in shell)--10 to 14 ounces

Large shrimp (unshelled)--10 ounces

Large shrimp (shelled)--3 ounces

Crab (in shell)--14 ounces to 1 pound

Oysters--6 to 12

Scallops--2 to 3

Mussels, Clams--14 ounces

Shellfish are highly perishable and it is best to use them within 24 hours of purchase. They need to be handled carefully before cooking to keep them fresh or alive. The most important factor is to keep them in a cold, moist environment. Keep fresh or thawed shellfish as close to 32°F as possible.

Store shrimp and shucked shellfish in a leakproof bag or covered container in the refrigerator. Store live shellfish in a shallow dish covered with a damp towel. If available, you may also want to keep the shellfish covered with seaweed. Never put live shellfish in an airtight container or fresh water, since they can suffocate and die. Some shells may open during storage. If so, tap them. They will close if still alive.

Frozen shellfish can be stored in the freezer for three to six months. The longer it is stored, the greater the loss of flavor, texture and moisture.

To clean clams, begin by scrubbing the shellfish thoroughly with a stiff brush under cold running water to eliminate seaweed, sand and filaments. Soak them in salted water (1/3 cup salt to 1 gallon water) for 20 minutes to remove any sand. Drain water; repeat two more times. Place clams on a tray and refrigerate them for 1 hour so that they relax, making them easier to open. To shuck clams, take a clam knife in one hand and a thick towel or glove in the other. With the towel, grip the shell in the palm of the hand. Keeping the shell level, insert the tip of the knife between the shells next to the hinge; twist to pry the shell until it snaps. (Use the knife as leverage, but do not force it.) Twist to open the shell, keeping it level at all times to save the liquor. Cut the muscle from shell and discard the top shell.

To crack a steamed crab, place the crab on its back. With your thumb or the point of a paring knife, pry off the "apron" flap (the "pull-tab" looking shell in the center) and discard it. Turn the crab over. Lift off the top shell and discard it.

Break off the toothed claws and set them aside. With the edge of a paring knife, scrape off the 3 areas of lungs and debris over the hard semi-transparent membrane covering the edible crabmeat.

Hold the crab at each side; break it apart at the center. Discard the legs. Remove the membrane cover with the knife, exposing the large chunks of meat; remove the meat with your fingers or a knife.

Crack claws with a mallet or knife handle to expose the meat.

To clean mussels, scrub with a stiff brush under cold running water. To debeard mussels, pull the threads from the shells with your fingers. Soak, relax and open following instructions for clams. Mussels die soon after debearding. Use them immediately.

To peel shrimp, remove the legs by gently pulling them off the shell. Loosen the shell with your fingers, then slide it off.

To devein shrimp, cut a shallow slit along the back of the shrimp with a paring knife. Lift out the vein. (You may find this easier to do under cold running water.) The veins of large and jumbo shrimp are gritty; they must always be removed. The veins of medium and small shrimp are not gritty and need not be removed unless you wish a more elegant presentation.

Most shellfish take very few minutes to cook and it is very important not to overcook them. If shellfish are cooked too long, they become tough and dry and lose much of their flavor. Heat precooked shellfish just to the desired temperature and not longer.

Lobsters are cooked according to weight. When boiling, the cooking time begins once the water returns to a boil. Cook lobsters from 10 to 18 minutes depending on their size.

1-pound lobster--10 minutes

1-1/4-pound lobster--12 minutes

1-1/2-pound lobster--15 minutes

2-pound lobster--18 minutes

Lobster is opaque when cooked. A bright red shell is not an indication of doneness, because the shell turns color when it comes in contact with boiling water. Crabs are cooked until their shells turn red and the meat is white. Shrimp turn pink and opaque when cooked.

Clams and mussels are done when their shells open. Remove them as they open and continue cooking until all are opened. Discard any that do not open. Shucked shellfish, such as clams, mussels and oysters, become opaque when done. Oyster edges start to curl. If they are overcooked, oysters shrink. Scallops become opaque and firm when cooked.