High-speed video and fancy math have overturned an old theory about how snapping shrimp make such a racket.
By quickly closing oversized claws while defending their territory, clusters of certain shrimp produce enough noise to interfere with the Navy’s sonar. But scientists didn’t agree about how the small creatures make such a din, says Detlef Lohse of the University of Twente in Enschede, the Netherlands. To make that snap, the shrimp squirt jets of water that form air bubbles, which then pop, Lohse, Michel Versluis, also of Twente, and their colleagues argue in the Sept. 22 Science.
Physicists refer to this bubble formation as cavitation, a process that can destroy ship propellers. “What I like most about our study is that this little animal uses cavitation and all that violent energy,” Lohse remarks. “Evolution has taken a weird direction.”
The idea that snapping shrimp, in the genera Alpheus and Synalpheus, get their sound power from popping bubbles has been floated before but never directly tested. In a 1998 survey of snapping shrimp noises, underwater-acoustics specialist Whitlow Au at the Hawaii Institute of Marine Biology at Kaneohe attributed the sound to slamming claw parts.
Lohse, a bubble specialist, was intrigued when a colleague reported a bubble zipping from a shrimp’s claw. Could the famous snap be a pop? After all, the sound of rain on a lake comes from the pop of the bubbles that form at impact.
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In the lab, Lohse and his colleagues set a video camera running at 40,500 frames a second to record a shrimp that they tickled with a paintbrush. They detected snaps that began some 700 microseconds after the crustacean’s claws closed. So much for the clattering claw-part theory, which Au, too, now rejects.
After combining video images with theoretical calculations of bubble behavior, the researchers became convinced that the snap is definitely a pop. They also say they can see where the cavitation occurs.
The oversize claw cocks open when two muscles pull the prongs in opposite directions. When a third muscle contracts, the high-tension claw slams shut. A bump on one prong’s inner edge plunges into a pocket on the other prong, squirting out water. Calculations suggest that the tiny jet moves at up to 30 meters per second.
Such speeds are so high that pressure drops below water’s vapor pressure, and bubble nuclei in seawater, ranging in diameter from a few to 100 microns, suddenly swell to 3 or 4 millimeters across. When the jet slows and pressure returns to normal, the bubbles implode.
The team’s film and its calculations have convinced the once-skeptical Au. “What they did was really outstanding,” he says.
Snapping plays a major role in the lives of these shrimp, notes Nancy Knowlton of Scripps Institution of Oceanography in La Jolla, Calif., a specialist on the genera. The hundreds of snapping shrimp species seek daytime burrows or crevices and then defend them furiously by snapping at intruders. Fish can easily gulp an exposed shrimp. “I’ve heard them still snapping inside a fish’s mouth,” Knowlton recalls.
The shrimp go to extremes for protection. In one species, a male and female wriggle into a burrow and excavate a cavity. As they grow, neither can get out, so they jointly defend their prison for the rest of their lives. In a handful of snapping shrimp species, members even defend the equivalent of a honeybee hive—a queen shrimp rules a colony of nonreproductive but snappy workers.
The snaps that make this highly territorial lifestyle possible are loud even to human ears, says Knowlton. On field trips when she brings shrimp back to camp, “they wake me up at night,” she says. When she snorkels into a shrimp-rich area, “it sounds like popcorn,” she adds.