Although sharks are classified as fish, they differ significantly in several respects from other freshwater and saltwater fish. Most significantly, a shark's skeleton is composed of lightweight, flexible cartilage, providing an advantage in hunting other marine life; other superclasses of fish have stable calcified skeletons. Additionally, sharks possess no swim bladder, the small organ that allows most fish to control their buoyancy; instead, a substantial liver filled with oil works to keep the sharks afloat.