Two Designs for Survival

BIOMECHANICS can help explain why organisms are shaped the way they are. Living in the wave-swept environment, a limpet, for example, would appear to have a better chance of surviving if it were ``built'' low to the ground. But as an engineering analysis shows, that's not necessarily true.

The diagram illustrates the tradeoff between lift and drag that determines the hydrodynamic force on two limpets. The heavy arrow shows the direction of the action.

The Lottia gigantea, or giant owl limpet, lies comparatively low, but it is subject to lift like that on an airplane wing.

By contrast, the taller Acmaea mitra, or white-cap limpet, is subject to higher drag as waves push relentlessly at its sides.

As a result, although the two limpets are shaped quite differently, the net force imposed by the ocean on the adhesive that attaches them to the rock is remarkably similar. Indeed, the ``streamlined'' giant owl limpet is actually under slightly more stress.