Cooking shellfish can be a tricky business, since you need to heat them sufficiently to destroy harmful organisms, but not so long as to toughen their flesh (shellfish can overcook in a matter of seconds). Whether you're sautéing, steaming, poaching, or stewing, here's how to judge the doneness of eight popular types of shellfish.
Shrimp. Shrimp will feel firm to the touch and their flesh with turn opaque. If they are in the shell, their color will change from grayish-green to brightpink or orange.
Scallops. Small bay scallops and larger sea scallops cook at different rates, but the visual test for doneness is the same for both: They will firm up and become opaque throughout.
Crabs. The shells of live crabs will turn from green or blue to scarlet, and the flesh will turn from translucent to opaque. In addition, the apron (a triangular flap of shell on the crab's belly) will loosen when the flesh is cooked. Soft-shell crabs will turn light red or brown and feel firm to the touch.
Lobster. A lobster shell will turn from green or blue to scarlet, and its flesh will turn from translucent to opaque. (You can also test lobster for doneness by tugging on one of the small legs—it should pull off easily.)
Clams. Clams cooked in the shell will open when they are done. Any clams that do not open should be discarded as this is an indication that they were not alive when they went into the pot. Shucked clam meats will turn opaque and firm-textured, and their edges will curl slightly.
Mussels. Mussels are done when their shells open. Any mussels that do not open should be discarded as this is an indication that they were not alive when they went into the pot.
Oysters. Whole oyster meats will become slightly opaque when they’re done, but the real clue to their doneness is that their edges will begin to curl.
Squid & octopus. Squid and octopus will turn opaque and be fork-tender. This can be hard to judge with thin rings of squid, but biting a piece will help.
Also see our slideshow: 10 Spectacular Shellfish Recipes.