The Perfect Limpet
Natural History, February 2001
It would seem axiomatic that well designed animals outperform badly designed ones and pass on to their offspring the genes that helped create that design. As good genes arise and spread, animal designs should theoretically shift from the flawed toward the better. So when a design is significantly inferior to what might seem best for a given situation, something interesting is probably going on. Stanford University’s Mark Denny recently investigated one such apparently poor design-that of the limpet.
Limpets are marine gastropods that live on rocky coasts and in tide pools. They excrete a gluelike substance to anchor themselves to rocks but can loosen their grip enough to slide around on their single “foot” and graze on algae. 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. But if the limpet has a rounded hump, the water may pull it off its rock in the following way: The presence of the limpet creates an interruption in the onrush of the wave, “squeezing” the water over the rounded top of the shell and forcing it to move faster, which lowers its pressure. 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. The difference in pressure-high beneath the shell, low above it-can create enough lift to suck the limpet right off its rock.
To measure the drag and lift experienced by limpets, Denny put acrylic models of their shells in a wind tunnel. (Technically, air and water are both fluids, and as such, they both flow. Air may be 800 times less dense than water, but it is easier to work with experimentally.) Some of the models had small holes to which Denny attached sensors that measured how pressure was distributed over the shell. Other models were rigged up with springs at their base to measure how much drag the wind imposed on them. And Denny tried out a range of different shell shapes-some with high peaks, some with low ones, some with central peaks, and others with peaks set off to one side.
Studying his results, Denny figured out the specifications for the perfect limpet shell: to get the best protection against the combined dangers of lift and drag, a shell’s height should be 53 percent of its length and its peak should be directly over the center of its shell. Yet real limpets are a long way from perfection. Most of them are fairly squat, with an average height-to-length ratio of only 34 percent. Seemingly making matters worse, their peaks are generally well off center.
To understand why limpets have such an imperfect design, Denny switched from the ideal to the real, making a close study of the owl limpet, Lottia gigantea. Found along the California coast, the owl limpet can reach up to four inches in diameter. Its height-to-length ratio is an embarrassing 25 percent, and the peak of its shell is perched at its front end. Pugnacious, it pushes away other limpets that trespass on its little patch of rock. However, faced with certain predators, such as starfish, it lifts up its shell and slides away as fast as it canó at a speed of one inch per second. On the rocky coastline near his lab, Denny measured how much force it took to pry a limpet off a rock. He also lassoed limpets with a loop of string and tugged on them to gauge how well the shells withstood drag.
Denny found that the most important factor determining whether a limpet gets washed away or not is how tightly glued to the rock it is. If an owl limpet is hunkered down when hit by a wave moving at eighty feet per second, it has a 91 percent chance of holding fast, but if it has the bad luck to be running away from a starfish, it has only a 0.5 percent chance of not being washed away. An optimal shell design doesn’t change the odds very much: a perfect owl limpet would have a 95 percent chance of holding fast if it was anchored, a 2.4 percent chance if it was on the go.
The minor risk posed by the owl limpet’s shape is probably offset by the advantages. The off-center peak of this creature’s shell allows the limpet to use it like a bulldozer to clear its territory of other animals. Fewer competitors mean more food, which may ultimately translate into more baby limpets. And together with the strength of its glue, the limpet’s habit of hunkering down when big waves start pounding its rock may help compensate for its “poor” design.
Like engineering, biomechanics is all about trade-offs. But nature has a special set of trade-offs that human engineers don’t have to worry aboutó when designing skyscrapers, they can be reasonably sure that other skyscrapers aren’t going to move in and try to push theirs off the block.
Copyright 2001, Carl Zimmer. Reproduction or distribution is prohibited without permission.