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In this issue |
Super Suction? (p. 539)
kathryn{at}biologists.com
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People have been tugging at limpets for hundreds of years to get to the bottom of their incredible stickability. As the tide drops, limpets exude a sticky mucous that glues them securely when high and dry. But in a rolling swell, the problems are different, with every wave threatening to dislodge the animal while feeding. Most people thought of them as sticking like suction pads. But suction pads only hold tight against a lifting force and are easily dislodged when hit by a glancing blow, like a wave. Watch a threatened limpet clamp its shell to a rock, and it becomes apparent that a limpet’s adhesion is more than simple suction. Ellem knew that if he could measure the force when he tried to wrench the limpet loose, and the force from the limpet’s foot as it resisted, then he could calculate the force of the shell grinding against the rock, to find out whether limpet’s are suction pads or gripping clamps.
Ellem went down to the seashore and prized some limpets free. He transported them back to the lab where he and John Furst had built a force-transducing cyber-rock to measure how tightly the limpet ‘hunkered’ down when a simulated wave hit.
First Ellem tested how tightly the molluscs could cling when he gave them a warning tap. Depending on much he disturbed the small creatures he measured forces from 2 N up to 25 N, which is quite something for animal that weighs less than 25 gm. But to realistically simulate the reaction to a breaking wave, Ellem had to design a piston system that would apply increasing forces to see high tightly the molluscs clamped down.
When a wave crashes across a limpet’s shell, most of the force is delivered within the first 100 ms. Ellem recreated the impact of a breaking wave, and found that the limpets reacted almost instantaneously as the cyber-wave rolled past, clamping down the shell with forces that exceeded the wave force by up to 50 %. More remarkably, the limpet’s response was much faster than a standard muscular response.
So a limpet’s body plays an active role in its adhesion to a rock surface; it pulls itself down and sticks by more then suction alone. Ellem hopes to integrate his findings into an all-encompassing hydrodynamic model to understand how the sea has affected the limpet’s natural distribution, and maybe explain how they arrived at their shape.
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