Whole fish, larger fillets, or steaks may be checked for doneness with an instant-read thermometer. Remove them from the heat source as their internal temperature begins to approach 140 degrees to protect them from overcooking. Allow fish to rest for two or three minutes before serving-the internal temperature will continue to rise an average of ten degrees as it sits.
The doneness of thinner fillets is more difficult to tell using a thermometer and calls for a more basic approach. Slit the thickest part of the fish with a thin bladed knife and peek inside. When the appearance of the fish changes from translucent to almost fully opaque and just begins to flake easily, remove it from the heat source and let it rest for a few minutes.
Shellfish, such as mussels or clams will cook and open after only three to five minutes of cooking. When most of the shellfish have opened, discard any that remain closed.
Shrimp and scallops are cooked when they have turned opaque, their texture changes from mushy to slightly firm, and shrimp take on an orange cast-about two to three minutes of cooking per side.
Lobsters and crabs are cooked when their shells turn a deep red and their meat changes from translucent to opaque. Large pieces of meat, such as lobster tails, will register about 140 degrees on an instant-read thermometer. Depending on the cooking method and size of the shellfish, lobster and crab may take anywhere from five to twenty-five minutes to cook.