In a culinary sense, shellfish are edible aquatic invertebrates. They have shells but are not actually fish. They fall into two main groups: crustaceans and mollusks. Crustaceans have segmented bodies and many legs; they include crabs, crayfish, lobster, shrimp, and prawns. Mollusks have unsegmented bodies and (usually) shells; they include univalve ("single shelled") organisms such as abalone; bivalve ("double shelled") creatures like clams, oysters, and scallops; and cephalopods ("head-body") like octopus and squid.