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The real story of the snapping shrimp.

One of nature's curiosities has been explained--and it is even more curious than it was before. Scientists have known for a long time about a special little shrimp, the snapping shrimp. It makes a big noise in the ocean. In fact, its snapping can be a nuisance for submarines and for dolphins that use the echoes of sonar to locate underwater objects.

Shrimp are common among the little animals of the world's oceans. Instead of a bony inside skeleton, they have an exoskeleton, which is a loose-fitting shell.

The snapping shrimp has two claws, one of them a giant claw almost as big as its body. The giant claw is not used for attack. Instead, it is designed so that one of the pincers can be snapped into a tight-fitting socket in the other pincer. When the claw snaps shut, it works like a water pistol. It sends out a jet stream of water. The giant claw also makes the loud snapping sound that gives the shrimp its name.

When scientists saw that jet of water, they could easily imagine how the claw might discourage a predator and even stun or kill little animals that become the shrimp's prey. You can see why the snapping shrimp is also called the "pistol shrimp."

Dr. G.E. Macginitie, a scientist famous for his studies of marine animals, once told about two pistol shrimp kept in an aquarium in his laboratory: "They were always ready to demonstrate their shooting ability for visitors. When seeking food, a pistol shrimp lies just at the entrance of its burrow with its long antennae extended to detect any passing movement. When a small fish passes by, the shrimp slowly creeps out, and the fish is paralyzed by a shot from the pistol hand. The shrimp then reaches out, grasps the stunned prey, and drags it inside the burrow."

How Does It Work?

Another marine scientist, Dr. Barbara Schmitz, watched snapping shrimp in action on the coast of Florida. She then brought some to her laboratory in Germany for study with her friend Jens Herberholz.

Using a small plastic tube, they could tease a shrimp to do its thing--snapping shut its giant claw to make its loud sound and water jet. By putting a drop of ink at the end of the tube, they could watch the jet shoot forward. They learned more by taking video recordings, and even more by using a high-speed video camera that took one thousand pictures a second.

On these pages, you can see drawings that show the water jet in action. The scientists could even measure the speed of the jet coming out of the claw. It started out fast, going almost sixty feet per second. But even at short distances the jet slowed way down. So the jet was not strong enough to stun the shrimp's prey.

How could the claw stun prey and make so much noise?

Dr. Schmitz's research drew so much attention that two scientists from Holland came to join her, bringing their special equipment for recording underwater sounds. Now the group could take extra-high-speed videos up close to the snapping claw while recording the sounds.

SNAP! It stuns its prey like THAT.

The video pictures showed that right after the claw closed, a bubble of gas formed at the edge of the claw. Then the bubble kept getting larger till it suddenly disappeared. Side by side with the video pictures, Dr. Schmitz could watch a record of the sounds. There was an expected sound--like a click--when the claw snapped shut and the water squirted out. The big sound came later, almost one thousandth of a second later, and it came just when the video showed the bubble vanishing.

Loud Bubbles

Now it was clear that the great sound power of snapping shrimp comes from collapsing bubbles.

The idea of loud sounds from collapsing bubbles may be a surprise to most of us, but it is well known to scientists and engineers who study moving fluids. They call it cavitation.

Here's how cavitation works. Water contains tiny bubbles of air. Water is also always under pressure, but that pressure decreases in a fast-moving flow of water. At very high flow, the pressure is so low that tiny air bubbles suddenly expand to become big ones. When the water flow slows down, the bubbles collapse. Bubble collapse is so rapid that it releases a whole lot of energy as a sound wave that's strong enough to punch tiny holes into metal. So cavitation is a common problem with propellers and pumps that move rapidly in water.

Cavitation is how the snapping shrimp does its thing. It uses bubbles caused by its water jet. Those collapsing bubbles release a shock wave of sound so intense that it can stun close-by prey.

That's the story of how science worked to find out about the snapping shrimp.
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Author:Myers, Jack
Publication:Highlights for Children
Date:May 1, 2003
Words:815
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