Natural History

February 2001

In the real world, striving for the ideal may not be worth it.

Story by Carl Zimmer - Illustration by Sally J. Bensusen

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.

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. (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 can-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
about-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.

Science writer Carl Zimmer is the author of At the Water's Edge and
Parasite Rex.

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