Dermal denticles cut down on drag when the shark is swimming. They also protect the shark from injury. Bony fish have gas-filled swim bladders that keep them afloat. Sharks don't have swim bladders. Because they don't have swim bladders, sharks must constantly swim.
Sharks have sharp, triangle-shaped teeth.
Their teeth grow in rows. Most sharks are meat-eaters, but the basking shark and the whale shark eat plankton. Rays Rays are closely related to sharks. They have flat bodies with eyes and mouths on their undersides. They have long slender tails.