For such a shrimp, Alpheus heterochaelis makes a big racket. Equipped with one snapper claw that can grow to half the shrimp's size, A. heterochaelis captures prey and blasts trespassers with jets of water by snapping the claw shut. Scientists have attributed the crackle of snapping shrimp colonies to claws banging together, but now a study reveals the shrimps' real noisemaker: bubbles.
Smaller than a finger, A. heterochaelis lives in warm, shallow seawater, often burrowing below coral rubble or among oyster clumps in tide flats. Its snapper claw looks like a mottled green boxing glove. Muscles on each side of the snapper claw slowly contract, cocking the claw open like a revolver--until an unfortunate little crab, for instance, triggers the claw to slam shut.
A few years ago, zoologist Barbara Schmitz of the Technical University of Munich in Germany noticed the curious flash of bubbles when she was videotaping shrimp. She teamed up with physicist Detlef Lohse of the University of Twente in the Netherlands, and they used a faster camera to capture the movements of shrimp tethered in a lab aquarium. The video shows an air bubble forming between a shrimp's closing claw and then blasting away along with the water jet before the claw shuts. Some 300 microseconds later, the bubble balloons as large as 7 millimeters in diameter and then shatters, broadcasting a loud snap, they report in the 22 September issue of Science. Snapping shrimp may be the first animals known to create these forceful "cavitation" bubbles, more commonly churned by the propellers of ships, says Lohse.
"This is one of those studies that makes you think, 'Damn, I wish I'd done that,' " says physicist Lawrence Crum of the University of Washington, Seattle. "What's remarkable," Crum says, "is that this shrimp can move its claw fast enough to create a vapor bubble." However, physicist Michael Buckingham of the Scripps Institution of Oceanography in La Jolla, California, wonders whether suction pad-style membranes on the back of the shrimp's claw might cause the bubbles.