Shrimps spew bubbles as hot as the sun

Pinkie-size marine crustaceans whose snappy noisemaking has already captivated scientists also stage some flashy pyrotechnics, researchers now find. While earlier experiments had shown that so-called snapping shrimp generate imploding air bubbles that make loud popping sounds (SN: 9/23/00, p. 199), a new study reveals that those collapsing bubbles emit flashes of light and may flare as hot as the sun’s surface.

Versluis/U. Twente

When a shrimp (bottom) slams a claw shut (top), a hot bubble forms and emits light too dim to be seen. Cloud of tinier bubbles (circled) reflects camera’s light. Schmitz/Tech. U., Munich

In the Oct. 4 Nature, Detlef Lohse of Twente University in Enschede, the Netherlands, and his colleagues present measurements of those light flashes. Using readings from a sensitive light detector called a photomultiplier tube, they offer the first evidence of a biological version of the phenomenon known as sonoluminescence.

That’s a process in which sudden collapses of gas bubbles in a liquid cause temperatures and pressures to soar inside the shrinking orbs. Under such extremes, the gases inside the bubbles momentarily incandesce (SN: 6/21/97, p. 391) and reach temperatures as high as 20,000C.

“For an animal to do that is pretty remarkable,” comments physicist Lawrence A. Crum of the University of Washington in Seattle.

Sonoluminescence results from cavitation–bubble formation in a liquid when its pressure dips below that at which the liquid would ordinarily vaporize, permitting the microscopic gas bubbles already present to expand. When such bubbles shrink suddenly as the pressure returns to normal, they can launch shock waves forceful enough to damage ship propellers, water pipes, and other equipment.

In lab research, scientists use sound waves to make the bubbles–hence the prefix sono- in the term sonoluminescence. Lohse and coauthors Barbara Schmitz of the Technical University of Munich in Garching, Germany, and Michel Versluis, also of Twente University, have wryly dubbed the shrimp-initiated sound-and-light show “shrimpoluminescence.”

Snapping shrimp, in the genera Alpheus and Synalpheus, inhabit mostly shallow, tropical and subtropical waters worldwide. The animals use snaps to fight rivals, find mates, and even stun prey.

Three years ago, Schmitz, a biologist, noticed flashes when the shrimps’ enlarged claws made the snapping sounds. She suspected cavitation-induced luminescence. After experiments using high-speed video cameras and hydrophones, she and the Dutch physicists reported a year ago that snapping does indeed produce cavitation. Now, experiments in darkness show that the cavitation, in turn, causes luminescence–but not the bright bursts that Schmitz first noticed while videotaping the animals.

Those proved to be due to the camera’s lights reflecting from other tiny bubbles the snaps also make. The luminescence was more subtle, detectable only with a photomultiplier tube. With it, the team recorded bursts of cavitation-induced light that are extremely brief and faint. In fact, those pulses produce too few photons for human eyes or probably any other eyes to see, Lohse notes.

That dimness suggests that the light has no biological significance, say Schmitz and other biologists. The flashes are most likely a byproduct of the snapping that’s central to the shrimps’ lives.

This lack of biological consequence may underscore how common cavitation-induced luminescence could be, suggests Kenneth S. Suslick of the University of Illinois at Urbana-Champaign.

Researchers have previously observed sonoluminescence outside the lab only in certain water pumps and in liposuction surgery using bubbles acoustically made, notes Seth J. Putterman of the University of California, Los Angeles.

Yet cavitation is widespread, Suslick notes, from ultrasound imaging to steam bubbles from deep sea vents (SN: 9/7/96, p. 156: Cavitation bubbles in synovial fluid may even explain the sound of “cracking” knuckles, he ventures. And if that’s the case, he says, “I’d be willing to bet pitchers of beer that cracking knuckles will also generate small amounts of luminescence.”

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