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. Many rays skim the ocean floor looking for food. Many rays have strong jaws that help them crush mollusks and crustaceans.