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06/01/92

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SHELL DESIGN HELPS LIMPETS SURVIVE LIFE ON THE EDGE

STANFORD - Living on the edge, in the violent verge of ocean and land, coastal organisms employ thousands of strategies toward a single goal: living to fight - and reproduce - another day.

Next door to the Monterey Bay Aquarium, at Stanford University's Hopkins Marine Station in Pacific Grove, a marine biologist studies how life persists, and even flourishes, in this constant battleground.

Despite intense elemental stresses, the coastal ecosystem is rivaled in biodiversity only by the tropical rainforests, said Mark Denny. The intertidal zone - described by John Steinbeck as "ferocious with life" - is "one of the most productive," Denny said.

Denny studies how the limpet - a common marine snail whose shell resembles a tiny volcano - adapts to its environment. Such basic research has improved scientists' understanding of animal behavior, enabled engineers more accurately to predict the movement of fluids and particles during large-scale mixing, and allowed researchers to infringe on Mother Nature's patents by copying the limpet's mucus to create adhesives and its shell to design stable, aerodynamic shapes.

Denny has built to simulate coastal forces a U-shaped wave machine - 18 feet high and 16 feet long - and a wind tunnel of comparable length. This summer, he will blast limpet shells with three-foot waves (creating pressure the equivalent to a human being hit with 10 tons),and 150 to 170 mph winds to determine how the shape of a limpet's shell helps it withstand such pressures.

Clinging tenaciously to rocky surfaces, limpets defy the mechanical forces of drag and lift that conspire to pry them from their beachfront homes. Drag acts in the direction of water flow; lift acts at right angles to it.

Water blasts past limpets at five yards per second, Denny said, causing the pressure surrounding the top of the limpet shell to fall. With little fluid movement under the shell, where the foot and guts seal out water, pressure there remains high.

"The stage is set for the imposition of lift," Denny said.

The same phenomenon rips roofs from closed-up houses during tornados, Denny said.

Nevertheless, despite pressures equivalent to those a human would experience amid 320 mph winds, a limpet stays put. That is partly thanks to muscles in its "foot" that create the pressure necessary to adhere to rocks and its slimy mucus, which develops a good seal.

However, the limpet's ribbed, conical shell may play the paramount role in adhesion by minimizing lift forces, Denny said.

That shell design was copied by California Institute of Technology engineers to produce a solar-powered electric car, the Sunraycer, in the 1980s. The 400-pound automobile won a 1,867- mile race across the Australian outback on the solar equivalent of 5 gallons of gas. It averaged 41 miles per hour.

Using computer simulations and wind tunnels, the engineers designed the limpet-shaped car for stable performance on outback roads, where wind gusts from passing trucks could cause the driver to lose control of the vehicle. By arranging the solar panels in circular tiers, like the ribs on a limpet shell, they were able to stabilize the design.

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This story was written by Dawn Levy, a science writing intern with Stanford News Service.

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