The top shell, called the carapace, of all species of crabs offers little flavor other than that from the greenish brown tomalley you will find tucked in the interior. The head sac attached to the carapace, behind the face of the crab, is inedible and should not be added to stock. So I remove the tomalley to add to the stock for extra flavor and discard the carapace. The body is filled with meat and is excellent added to stock, especially if it is not picked of all its meat. The shells from the legs add good flavor whether they are picked or not.
If you are using picked-over shells, they will already be broken up enough to be used for stock with no further preparation. If you are using whole crabs, pull off the top shell (carapace) and remove and save the tomalley. Discard the carapace. Break off the legs and roughly chop them, in order to extract maximum flavor. Cut the body into 4 or more pieces about an inch in size.
Besides the crab, only a few vegetables, herbs, and spices are needed to make a great stock. More assertive seasonings are best added later, directly to the chowder. Use this recipe as a guideline, adding more onions or celery to your taste, but do not increase the amount of thyme or bay leaves.
For equipment, you need a 6- to 8-quart stockpot, a ladle, and a fine-mesh strainer; I like to use a medium-weight Chinese cleaver when chopping up crabs, but a large chefs knife will do.
