The limpet's shell, a low cone that looks like a gently sloping hill, provides protection from crabs, birds, and other predators. It also helps the animals survive the waves that regularly slam against them at speeds greater than eighty feet per second. The shape of a limpet's shell has a great deal to do with whether the animal remains securely attached to its rock or is ripped off and thrown onto dry land or into the waiting tentacles of a hungry sea anemone. There are two ways a wave can dislodge a limpet. If the limpet's shell is steep-sided, the drag of the water against the sides of the shell will force it in the direction of the wave.
Water flowing around the base of the shell moves more slowly, raising the pressure.
This water pushes against the limpet under its shell, producing high pressure in its body as well. For a limpet, the best way to avoid being swept from a rock by powerful waves is to have a shell with a height 53 percent of its length and a peak directly over its center. In reality, however, most of these mollusks, like the owl limpet above, make do with a squat shell and an off-center peak. To measure the drag and lift experienced by limpets, Denny put acrylic models of their shells in a wind tunnel. Some of the models had small holes to which Denny attached sensors that measured how pressure was distributed over the shell.