The most commonly available types of crab, depending on where you live, include: blue crabs from the East Coast, Dungeness from the West Coast, king crabs from Alaska and stone crabs from Florida.
Blue crabs are small, sold live and usually boiled, steamed or turned into soup. During spring, when the crabs molt, you can find soft-shell crabs sold cleaned. The entire soft-shell crab is edible, and it's superb when lightly floured, sautéed and served with lemon. Fresh-picked blue-crab meat is sold as "lump," meaning large chunks (the most desirable and pricey), or "flake," indicating smaller bits.
Dungeness crabs are larger than blue crabs, and they are usually sold cooked, often frozen. King crabs can weigh up to 25 pounds. The legs are usually sold cooked and frozen.
Black-tipped stone crab, a delicacy of Floridian waters, is hardly ever sold whole. When captured, one of the claws is broken off and the live crab is thrown back into the water to regenerate another smaller claw. The claws are usually sold cooked and frozen.
The high demand for American lobster has resulted in higher prices, smaller lobsters and smaller harvests. They are found in shallow waters from Canada to North Carolina, with lobsters from colder waters considered to be of higher quality. Lobster season in New England runs from July to October.
The minimum weight for live lobsters sold on the market is 1 pound, but the most common weight sold is 1 1/2 pounds. The proportion of meat to shell is lower for small lobsters, so larger lobsters cost more per pound.
The quality of live lobster deteriorates as it sits in a tank, so choose a well-trafficked market and look for moving, active lobsters. The tail should snap back when straightened, and the shell should be hard and thick. They can be stored live for up to one day in a box or shallow pan set in the refrigerator, covered by a damp cloth, seaweed or layers of damp newspaper.
Lobster meat tastes mild and sweet with a meaty texture. Live lobsters are usually boiled or steamed whole. They can also be split in half lengthwise before grilling or roasting, or cut into pieces and sautéed. Frozen lobster is the tails of clawless varieties, like spiny or rock lobsters, and can often be used in recipes that call for live lobster. Other crustaceans, like shrimp or crayfish, can also be substituted.
Mussels don't have the same cachet as other shellfish, but when steamed in a mixture of white wine, garlic, parsley and butter, they make a fabulous dinner. Wild mussels are full of sand and dirt, and a chore to clean. Farm-raised mussels are significantly cleaner and just as flavorful.
You should buy 1 to 1 1/2 pounds of mussels per person for a main-course serving. The most common type is the black-colored "blue mussel," but green-shelled New Zealand mussels are popular, too.
Mussels are sold live and their shells should be tightly closed, but some may "gape" open slightly. Try pressing the shell shut. If it stays closed, the mussel is still alive and safe to eat, but if it opens again, throw the mussel away.
Despite the many names (bluepoint, Wellfleet, Kumamoto and Hamma Hamma), chances are you're choosing between two main oyster species: the Atlantic oyster, grown up and down the Eastern Seaboard, and the Pacific oyster. The European flat, or Belon oyster, and the Olympia oyster make up the two additional species grown in America. They are both harder to find.
Oysters take their names from the specific areas in which they grow. Because the water it inhabits greatly affects an oyster's taste, its origin is important. For example, a Wellfleet oyster tastes different from a Chatham oyster even though they both come from the waters off Massachusetts.
Oysters must be alive when you buy them, with shells tightly closed. They should be stored flat in the refrigerator to prevent their liquor from leaking out. Your fishmonger may be able to shuck oysters for you, but eat them as quickly as possible once they are out of the shell. You can also learn to shuck your own oysters.
Oysters taste delicious raw, stuffed or baked. There is no substitute for an oyster, but hard-shell clams and mussels may be substituted in some recipes.
Scallops are the sweetest and creamiest of shellfish. They are shucked at sea because they spoil quickly, and we buy the large, meaty muscle that opens and closes the shell. Sometimes the roe is still attached (it is edible and a delicacy).
Sea scallops are large and available year-round. Bay scallops and calico scallops are smaller varieties. Bay scallops are only available in winter and very expensive, because they're harvested from a small area of the Atlantic seaboard. The cheaper calico scallops are less desirable, since they toughen quickly in cooking and aren't as sweet.
When possible, buy "dry scallops" that have not been treated with a phosphate preservative. Treated scallops absorb water (making sautéing almost impossible) and have an off taste, so avoid scallops that are very white and sitting in liquid. Scallops should be ivory to beige in color.
Scallops are superb sautéed, grilled, roasted, poached and steamed. Substitute shrimp or lobster, or firm white fish such as monkfish.
Most shrimp is farm-raised and frozen, guaranteeing good quality and an abundant supply.
Shrimp is sold by size: small, medium, large and jumbo, or by number of pieces per pound. A good medium-sized shrimp will give you about 20 to 30 pieces per pound. Shrimp sold in the shell will have the best flavor, but you can also buy them with heads removed, shelled and deveined, or even fully cooked and ready to eat.
Almost any preparation method will work, but it's important to note that shrimp will cook quickly and dry out if overcooked. Crayfish or lobster can be substituted.