Secret Behind Shrimp's Snap Explained

Sept. 22, 2000 -- Like a chorus of chattering castanets, theunderwater drone of thousands of snapping shrimp can be so intensethat submarines use the cacophony to hide from sonar.

But how do marine animals so small make a racket so loud?Scientists have long been puzzled, but a group of Europeanresearchers have found the answer: The shrimp make bubbles thatcollapse with a pop powerful enough to kill small prey.

Snapping shrimp are 2-inch-long creatures equipped with a smallclaw and a huge, outsized claw, almost half the animal’s length.The shrimp prowl the shallow waters of tropical seas with the bigclaw cocked, ready to seize a meal.

When the big claw closes at lightning speed, there is a sharpclicking sound. If there are enough shrimp in a school, the soundbecomes rather like that of the crackling of burning dry twigs.

Pop From the Water

Clusters of tens of thousands of shrimp can make enough noise“to disturb underwater communications,” said Detlef Lohse, aphysicist at the University of Twente in the Netherlands.“Submarines have used colonies of these shrimp to hide in theoffshore waters of the United States.”

Lohse said scientists had assumed that the clicking sound camewhen the two parts of the shrimp claw slammed together, rather likethe snap of fingers.

But, as he and his colleagues report today in the journalScience, the sound actually comes from a process called cavitation,an action caused by high speed motion through water. Cavitation wasfirst recognized in 1916 when researchers discovered it was causingship propellers to pit and scar.

“This shrimp is the only known natural creature that cancreate this phenomena,” Lohse said.

The European researchers discovered the snapping shrimp secretwhen they put some of the creatures in a water tank equipped withultra high speed cameras, capable of taking 40,000 frames persecond.

“We tickled them to make them close their claws,” Lohse said.

He said the photos show that when the shrimp closes its clawvery rapidly, it creates a high speed water jet, moving at almost70 miles per hour. The jet causes a sharp and brief drop in waterpressure and instantly a bubble is formed and collapsed.

“A microscopic bubble grows to about 4 millimeters [a littleover 1/8 inch],” Lohse said. “When the pressure returns to normal,the bubble collapses and that makes the sound.”

Extremely Fast Snap

The time between the claw closure and the collapse of the bubbleis 700 microseconds, he said. A microsecond is one-millionth of asecond.

Collapse of the bubble also sends out a shock wave that, on avery small scale, is very powerful, he said.

Lohse said laboratory studies have shown that the temperaturesinside of cavitation bubbles can soar to more than 25,000 degreesand create a shock wave that may equal 14,000 pounds per squareinch. The energy release is very short, but intense, he said.

“It can be intense enough to melt steel,” he said.

The shock wave created by the snapping shrimp is not thatpowerful, Lohse said, but it is enough to provide the shrimp with ameal. The animal uses the shockwave to stun worms and other prey,he said.