Snapping shrimps rely on cavitation, a process that is best known for causing damage to ships, to stun and kill their prey, according to a team of researchers lead by Detlef Lohse, a physicist at the University of Twente in The Netherlands (M Versluis et al 2000 Science 289 2114). The mechanism exploits Bernoulli's principle - the fact that the pressure inside a fast-moving fluid decreases.
Scientists originally thought the shrimps’ snapping sound was produced by mechanical contact between opposing sides of the claw. But Barbara Schmitz, a biologist at the Technical University of Munich, and Lohse noticed during other studies that bubbles formed near the shrimps’ closing claws. They suspected that when a shrimp’s claw snaps shut, a jet of water is forced out at such a high velocity that the pressure drops below the vapour pressure of water. The reduced pressure inside the water jet then causes the tiny air bubbles that exist in seawater to expand rapidly. As the water pressure returns to its normal level, the air bubbles implode and generate a shock wave that is sufficient to stun or kill nearby small prey.
To test their theory, Lohse, Schmidt and co-workers at Twente and the University of Marburg in Germany tethered seven shrimps inside an aquarium and gently nudged their claws to persuade them to snap shut. A hydrophone recorded the sounds emitted and a high-speed camera observed the behaviour of the bubbles. The team’s suspicions were confirmed when they found that the main peak of the ‘snap’ always coincided with the collapse of the cavitation bubble, not with closure of the claw. Lohse would now like to know if the collapsing bubbles can also produce light, a phenomenon known as sonoluminescence. “It would be neat if light came out of the bubble,” he told PhysicsWeb. “We will have a look with a photomultiplier out of curiosity”.
Snapping shrimp usually exist in such large colonies that there is a permanent background noise in much of the world’s shallow oceans. The noise level has severely limited the use of sonar for scientific and military applications, and covers a frequency range so broad that it is impossible to filter out.