Sleuths Solve Case of Bubble Mistaken for a Snapping Shrimp
By KENNETH CHANG
Published: September 26, 2000
How does a snapping shrimp snap?
For a long time, biologists thought they had a simple explanation. Each snapping shrimp has one oversized claw that cocks open while a muscle inside the claw pulls taut like the spring of a mousetrap. Then, suddenly, the claw snaps shut in less than a thousandth of a second. The impact of shell hitting shell, the simple explanation went, produced the loud snap.
Wrong. The answer, it turns out, is bubbles.
Once again, scientists have discovered that in the ocean, what meets the ear is not always what first meets the eyes.
A decade ago, scientists found that most of the noise in water pelted by raindrops is not the sound of the raindrops hitting the water but rather vibrations of microscopic bubbles created by the impacts. Earlier this year, researchers reported that even snowflakes falling on water created shrieking bubbles.
In videotapes of snapping shrimp, Dr. Barbara Schmitz, a professor of zoology at the Technical University of Munich, noticed a couple of frames showing a bubble near the shrimp's claw as it snapped shut.
She later attended a talk about sonoluminescence, a phenomenon in which imploding underwater bubbles heat trapped air to 25,000 degrees and emit flashes of light.
Afterward, she mentioned her shrimp and the bubbles she had seen to the speaker, Dr. Detlef Lohse, a professor of physics at the University of Twente in the Netherlands.
''We had long discussions afterward during dinner,'' Dr. Schmitz said.
''When I heard the bubble had been seen,'' Dr. Lohse recalled, ''I immediately associated the sound with the collapse of the bubble.''
Unfortunately, although Dr. Schmitz's high-speed video camera snapped 1,000 frames a second, already more than 30 times as fast as a usual movie camera, it was still not fast enough to see what was happening.
Dr. Lohse brought the shrimp -- a species known as Alpheus heterochaelis that is found along the Atlantic coast of North and South America -- back to his physics laboratory and videotaped them with a camera that takes 40,500 frames a second.
The ultra-high-speed photography shows the claw snapping shut in 0.65 milliseconds and squirting out a jet of water traveling at 65 miles per hour. ''It's as fast as a car on an American highway,'' Dr. Lohse said.
Because faster-traveling fluids exert less pressure than slower ones, microscopic air bubbles in the water expand from a few millionths of an inch across to an eighth of an inch in size as the jet shoots out of the claw.
Then, as the jet slows, the water pressure increases, and the bubble collapses within a millisecond, producing the snapping sound.
Dr. Lohse said he planned to see if the collapsing bubble also produced a flash of light.
The original explanation, that the snapping sound comes from the impact of the two sides of the claw banging together, fails entirely. In the sound recordings, ''there is no single hint that you can also hear the closure of the claw itself,'' Dr. Lohse said. ''We can't detect it.''
The results, described by Dr. Lohse, Dr. Schmitz and their colleagues in the current issue of the journal Nature, amazed both biologists and physicists.
''I always assumed, like everyone else, it was the claw coming down,'' said Dr. Roy E. Ritzmann, a professor of biology at Case Western Reserve University in Cleveland, who in the 1970's described how the shrimp's fast-closing claw mechanism worked.
Dr. Lawrence A. Crum, who performed the research on the raindrop microbubbles in 1989, described the shrimp findings as ''fantastic,'' saying the study explained how the small crustaceans produced such a loud sound.
Colonies of snapping shrimp often produce enough racket to thwart ship and submarine sonar.
''It's one of those things I wish I had done,'' said Dr. Crum, a principal physicist at the applied physics laboratory at the University of Washington in Seattle. ''When you see it, it's a simple obvious result.''
For the shrimp, the snapping noise is of no use -- they cannot hear -- but it might help get them dinner. To hunt, a snapping shrimp moves within a couple of millimeters from a small crab, worm or fish and squirts a jet of water. The energy released by the collapsing bubble, also known as cavitation, probably helps stun or kill the prey.
''As far as I know,'' said Dr. Deforest Mellon, a professor of biology at the University of Virginia, ''it's the first time cavitation has been seized upon by an animal as a offensive or defensive weapon.''
Photo: The snapping shrimp, about two inches long, produces noise through microscopic bubbles. (R. Streif)