Crab, like lobsters and shrimp, are crustaceans. Hundreds of varieties of crabs live in the world's waters. In the United States, a crab's popularity is determined by its local availability: Along the Atlantic and Gulf coasts, blue crabs are eaten most often; in southern Florida, stone crabs are considered delicacies; along the Pacific coast from southern California to Washington, the Dungeness is the crab of choice; and in Alaska, king crab and snow crab reign supreme.
Like lobsters, crabs must be kept alive until cooking, after which their shells turn bright red. They are a little trickier to eat than lobsters, although in crab-eating regions such as Charleston and Maryland's eastern shore, folks think nothing of the messy task. In other parts of the country, many people prefer buying the sweet, velvety crabmeat already cooked and removed from the shell, although the flavor will be better if you extract the meat yourself. On the West Coast, Dungeness crab is usually sold cooked but still in the shell, although it is possible to buy the giant shellfish live for cooking at home. Alaskan king crab and snow crab are nearly always sold cooked and frozen except in Alaska, where they can be bought frozen.
Live hard-shelled crabs should move their claws with some vigor when poked. Live soft-shelled crabs should look moist and soft, with no hint of a hardening shell. If a crab's claws drop when you lift it, reject it. Ideally, buy them immediately before you plan to cook them, and go directly home from the market. The only exception is if a fishmonger will pack the crabs in seaweed and then in special boxes for transport.
Cooked crabmeat usually is pasteurized and sold in cans or vacuum-packed plastic bags. It is commonly served cold, in salads or with cocktail sauces, as heating diminishes its flavor, although it is also used to make crab cakes. Some fishmongers sell crabmeat freshly cooked and unpasteurized, packed in plastic tubs. This is usually meat from blue crabs. Lump crabmeat, also called backfin, or jumbo, is the most desirable, as it is white meat taken from the center of the body and is in the largest pieces. Flake crabmeat includes light and dark meat from the center and legs and is in smaller pieces, although it's still tasty. If possible, avoid cooked crabmeat that has been frozen.
Although it is always best to cook live crabs on the day you purchase them, properly packed live crabs can be refrigerated for up to 2 days. Lay them in a shallow bowl and nestle the bowl in a larger one containing ice. Cover the crabs with a damp kitchen towel or with wet seaweed, if it came with the crabs. Replace the ice as needed. Properly stored, the crabs should still be alive after a day or so. Discard any that are no longer living.
Freshly cooked crabmeat will keep in the refrigerator for 2 days; if the crabmeat has been frozen and thawed, eat it on the day you buy it. Pasteurized crabmeat packed in vacuum-sealed bags will keep in the refrigerator for 3 to 4 weeks. Frozen crab will keep in the freezer for 4 months; thaw in the refrigerator.
Adapted from Williams-Sonoma Kitchen Companion: The A to Z Guide to Everyday Cooking, Equipment and Ingredients (Time-Life Books, 2000)