Image: Michel Versluis

Since the 1940s, researchers have been interested in snapping shrimp. These curious marine creatures sport a giant claw (right) that makes a sharp sound when it snaps shut. Colonies can produce such an underwater din that it deafens anyone trying to scan the seas with sonar. During World War II, in fact, the constant crackling from these shrimp sometimes foiled military attempts to sound out hostile submarines.

For years, the assumption was that the sound occured when the moving part of the claw hit its stationary mate, like crustacean castanets. But new research has shown that that isn't so. A paper in today's issue of Science reveals instead that cavitation--a physical process that has long plagued ships' propellers--is to blame.

Cavitation occurs when liquid moves above a certain speed and, as a result, experiences a drop in pressure known as Bernoulli's principle. This phenomenon allows tiny air bubbles in the fluid to swell. And when the fluid slows and the pressure again rises, the bubbles implode--generating a shock wave and an accompanying sound. Scientists only made the link between the shrimp and cavitation at a recent conference when biologist Barbara Schmitz of the Technical University of Munich heard Detlef Lohse of the University of Twente in the Netherlands speak on sonoluminescence, the process by which sound prompts bubbles to emit light.

Back in Schmitz's shrimp lab, she and Lohse--together with Twente scientists Michel Versluis and Anna von der Heydt--tied seven shrimp next to a hydrophone and in front of an ultra high-speed camera, taking 40,000 frames per second. The resulting video and audio showed that the sound came about well after the claw had snapped. They believe that a tooth-shaped peice on the moving part of the claw plunges through a hole in the stationary part, shooting out a jet of water fast enough to cause cavitation. Simulations backed the idea up.

Knowing how the shrimp snap, the researchers also explain why. The shrimp use the sound to communicate with one another and to defend their territory. In addition, they take advantage of the damage cavitation can do to stun their prey.

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