Like a chorus of chattering castanets, the underwater drone of thousands of snapping shrimp can be so intense that 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 European researchers have found the answer: The shrimp make bubbles that collapse with a pop powerful enough to kill small prey.
Snapping shrimp are 2-inch-long creatures equipped with a small claw and a huge, outsized claw, almost half the animal’s length. The shrimp prowl the shallow waters of tropical seas with the big claw cocked, ready to seize a meal.
When the big claw closes at lightning speed, there is a sharp clicking sound. If there are enough shrimp in a school, the sound becomes 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, a physicist at the University of Twente in the Netherlands. “Submarines have used colonies of these shrimp to hide in the offshore waters of the United States.”
Lohse said scientists had assumed that the clicking sound came when the two parts of the shrimp claw slammed together, rather like the snap of fingers.
But, as he and his colleagues report today in the journal Science, the sound actually comes from a process called cavitation, an action caused by high speed motion through water. Cavitation was first recognized in 1916 when researchers discovered it was causing ship propellers to pit and scar.
“This shrimp is the only known natural creature that can create this phenomena,” Lohse said.
The European researchers discovered the snapping shrimp secret when they put some of the creatures in a water tank equipped with ultra high speed cameras, capable of taking 40,000 frames per second.
“We tickled them to make them close their claws,” Lohse said.
He said the photos show that when the shrimp closes its claw very rapidly, it creates a high speed water jet, moving at almost 70 miles per hour. The jet causes a sharp and brief drop in water pressure and instantly a bubble is formed and collapsed.
“A microscopic bubble grows to about 4 millimeters [a little over 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 bubble is 700 microseconds, he said. A microsecond is one-millionth of a second.
Collapse of the bubble also sends out a shock wave that, on a very small scale, is very powerful, he said.
Lohse said laboratory studies have shown that the temperatures inside of cavitation bubbles can soar to more than 25,000 degrees and create a shock wave that may equal 14,000 pounds per square inch. 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 that powerful, Lohse said, but it is enough to provide the shrimp with a meal. The animal uses the shockwave to stun worms and other prey, he said.