Sharks have cartilaginous skeletons, gill slits, and pectoral fins. They are apex predators including great white, tiger, blue, and hammerhead sharks. Sharks are found in a wide range of aquatic habitats from shallow coastal waters to deep ocean floors. Reproduction varies between ovoviviparity in great white sharks, oviparity in catsharks, and viviparity in hammerheads. Sharks eat a variety of prey from fish and crustaceans to marine mammals and other sharks. Great white, tiger, and bull sharks occasionally attack humans.