Any aquatic mollusk , crustacean , or echinoderm that has a shell.
mussels , scallops , and clams rank among the most commercially important. Certain gastropod mollusks, such as abalone , whelk , and conch , are also marketed. The main crustaceans are shrimp , lobster , and crab . Among echinoderms, {{link=sea urchin">sea urchins and sea cucumbers are locally popular. After being harvested, all shellfish are highly perishable. Many types are cooked live to protect the consumer against the effects of spoilage.