Please read About Cooking Fish in Liquid; for much of that advice appies to shellfish as well. But for many shellfishΓÇöespecially soft-shell clams, mussels, and lobstersΓÇöcooking in or over liquid is the easiest and often best choice. These foods are so flavorful that they can be steamed with no seasonings whatsoever and still produce a superb broth. Sometimes their poaching or steaming liquids are seasoned, and sometimes not; but in either case, they should never be discarded. The broth from steaming clams, mussels, or lobsters (or even a broth made from simmering shrimp shells) can be used in place of fish stock in any recipe or used on its own to make a wonderful risotto or the base for a fish soup or stew. See A Note About Liquids for Cooking Shellfish.
The key to successful cooking of unshucked clams, oysters, and mussels in liquid is thorough cleaning. There are few things worse than a gritty mussel or a muddy broth, and both can be avoided by diligent cleaning. Wash soft-shell clams and mussels as thoroughly as you would salad greens: in repeated changes of water, until it looks clear. Scrub hard-shell clams and, especially, oysters until their shells are free of mud and grit.
As noted elsewhere, clams, mussels, lobsters, shrimp, and some other shellfish clearly signal you when they are fully cooked. Clams and mussels, for example, pop open; shrimp turn pink. But some shellfish, such as scallopsΓÇöwhich are already shuckedΓÇödo not have these built-in indicators. And although steaming and poaching are fairly gentle cooking processes which help seafood retain its intrinsic moisture, they are also fairly quick cooking processes, and overcooking is possible.
