Some of the most elaborate seafood dishes are made by cooking different shellfish, each in the optimum way, and then extracting and combining their flavors. A variety of shellfish prepared this way creates deeply complex flavors redolent of the sea but not of any single creature.
Some combinations are simpler than others because the cooking methods are the same or similar. Mussels, clams, and cockles, for example, can all be steamed together; they are just added to the pot at different times. Combining mollusks with crustaceans is more complex, since the cooking methods are different. But it is still fairly easy to steam open the mollusks and then use the liquid they release to steam or stew crustaceans, such as lobster or crayfish.