Bony fish have swim bladders of gas that help them to keep afloat and rest at any level in the water without effort. But the shark, with its body of cartilage instead of bone, has no swim bladder. If it stopped moving for a moment, it would drop to the bottom of the sea.
One species has given up the endless struggle to keep afloat. The rays save their energy and live on the sea bed, eating molluscs and various crustaceans.
Manta rays, however, skim along just beneath the surface, supporting themselves on their enormous 'wings', which can be several meters across.