Snapping Shrimp Stun Prey with Flashy Bang

By John Roach
for National Geographic News
October 3, 2001
Among the fascinating creatures of the deep is a finger-size shrimp with
an oversize claw—resembling a boxing glove—that it uses to
stun its prey by snapping the claw shut. The snapping produces a sharp
cracking sound.

When colonies of the shrimp snap their claws, the
cacophony is so intense that submarines can take advantage of it to hide
from sonar.

A year ago, a team of European scientists revealed that the sound is caused by the bursting of a bubble that forms when a shrimp snaps its claw shut. Now, the team reports that the bubble emits not only sound but a flash of light—indicating the extreme temperature and pressure inside the bubbles before they burst.

What this light part does is highlights the extreme conditions achieved at collapse, said Detlef Lohse, a physics professor at the University of Twente in the Netherlands.

Lohse and his colleagues Michel Versluis, also from the University of Twente, and Barbara Schmitz of the Technical University of Munich in Germany explain the flashes of light associated with the bubble collapse in the October 4 issue of Nature.

Flashy Bubbles

The shrimp, Alpheus heterochaelis, is a dirty-green crustacean that prowls the shallow waters of tropical seas. It has two claws', one resembling an oversize boxing glove, which it uses to stun prey, such as small crabs, by snapping the oversize claw shut.

Lohse and his colleagues reported in the September 22, 2000, issue of Science that the stunning snap comes not from the clap of the claws coming together but from a bubble generated by the claws' rapid closing motion.

When the claw snaps shut, a jet of water shoots out from a socket in the claw at speeds of up to 62 miles (100 kilometers) an hour, generating a low-pressure bubble in its wake. As the pressure stabilizes, the bubble collapses with a loud bang.

The whole process, which was recorded with the use of high-speed cameras and sound equipment, occurs within 300 microseconds.

Now, using a device that counts photons, Lohse and his colleagues recorded a flash of light that occurs when the bubble collapses.

The flashing phenomenon is thought to be similar to sonoluminescence, in which bubbles that are in a liquid driven by a strong sound field emit light. The researchers have dubbed the shrimp activity shrimpoluminescence.

In sonoluminescence, the peak intensity of the emitted light is at a short wavelength. This indicates that the temperature inside the bubble is at least 10,000 degrees Kelvin (18,000 degrees Fahrenheit).

The researchers say the light emitted from the snapping shrimp's bubbles suggests that the temperature inside the bubbles must be at least 5,000 degrees Kelvin (8,540 degrees Fahrenheit) at the time of collapse. Otherwise, we wouldn't see it, said Lohse.

He compared the heating inside the bubble at the time of collapse to that of a bicycle pump when it's being used to pump a tire. When you pump your bike to get air, you feel the pump is getting hot, he said. It is getting hot because you pump fast and the heat generated at compression cannot escape.

"The same thing is happening here," he added. "The bubble is compressed and getting hot on such a fast time scale that the heat cannot escape."

Lightning Fast

The researchers recorded the flash of light that occurs when a bubble collapses as well as other dim flashes of light, which are thought to be associated with bubble fragments. The mechanism responsible for the secondary flashes isn't clear.

The duration of the flash that occurs when the main bubble collapses is extremely short, lasting no longer than 10 nanoseconds and as short as 300 picoseconds, said Lohse. A picosecond is one trillionth of a second, a time scale beyond the measurement capability of the researchers' tools.

The researchers were not able to see the flashes with the naked eye. Lohse doesn't rule out, however, the possibility that they might be visible to a person very close to the shrimp when the bubble bursts and whose sight is well adjusted to the underwater darkness.

The flashes appear to have no biological significance, the researchers say, but are a by-product of the bubble collapse, which the shrimp uses to stun or kill its prey.

But light emission is nonetheless indicative of the extreme conditions inside the bubble at collapse and therefore demonstrates the violence of the event, the researchers conclude in their scientific paper.

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