Never cut a raw clam or oyster. And only in an informal setting should you drain the juice from the shell into your mouth; do so as unobtrusively as possible. As for oyster crackers, there are two schools of thought about whether it's permissible to drop them into the sauce and retrieve them with a fork. Taste dictates.
For steamed clams, lift both halves of the fully opened shell (if the shell doesn't open during cooking, don't eat what's inside). Separate each clam by pulling it out by its neck. Discard the neck sheath and, holding it by the neck, dip it first into a cup of broth and then into melted butter, and eat it in one bite, neck and all. Pile all the empty shells on a second plate. You may choose to drink the cup of broth. The procedure is essentially the same for steamed mussels, though you don't have to worry about a neck and neck sheath.
For mussels cooked in a souplike sauce (moules marini�res, for example), lift each shell and extract the mussel with a seafood fork; alternatively, you can eat it right from the half shell along with its juice. The extra broth can be sopped up with fork-speared bread or eaten with a spoon as a soup.
Fried clams, oysters, scallops or shrimps can be cut with a fork and eaten.
Fantail shrimp (fried Oriental style) can be lifted with your hands by the tail, dipped into a sauce and then eaten. However, leave the tail.
With hard-shell crabs, first pull the legs from the body with your fingers and then suck out the meat (as noiselessly as possible). The remainder of the body can then be turned on its back and the meat removed with a fork or pick.
Crab claws served as hors d'oeuvres are to be picked up by the shell with your fingers, dipped in sauce and sucked out.
Soft-shell crabs are meant to be eaten in their entirety, both crab and shell, with a knife and fork.
Next, break the tail away from the body. If the tail has already been split, the meat can be easily removed. If it hasn't been split, just break off the little flaps and push through to get the meat out in one piece. Cut this meat with a knife and fork, dip and eat. The legs are then twisted off with your hands, and the meat is sucked out. Finally, the tomalley (green liver) of a male lobster can be eaten, as can the coral (roe) of the female.
When the shells have cooled a bit, you may lift them in your hand, tilt them into your mouth, and enjoy the garlic butter and juice. You may also spear a piece of bread with your fork and use it to sop up the juice.