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Patrick Guilfoile column: An incredible hammer on a shrimp

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Peacock mantis shrimp are ocean-dwelling creatures that live in coral reefs in the Indian and Pacific oceans. They range in size from one to seven inches long and are brightly colored. Some observers have described these shrimp as resembling “armored caterpillars.”

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Although they can be a colorful addition to the menagerie in salt-water aquariums, their presence often poses problems. Peacock mantis shrimp feed on snails, shellfish, and fish, so they can deplete an aquarium of its other inhabitants. These creatures’ unique appendages are another challenge for aquarists. Their powerful appendages act as clubs and can smash the shells of snails and mussels as well as the skulls of fish. They are strong enough to break glass aquariums, leading to the recommendation that these shrimp be housed in acrylic tanks.

Previous research showed that the shrimp’s small club (typically about 0.2 inches across) can generate more than 150 pounds of force. It can smack an object at more than 50 miles an hour, generating an impact similar to that of a .22-caliber bullet.

A peacock mantis shrimp can use its club thousands of times before it is replaced during molting; this drew the interest of researchers from Harvard and several other Universities. They were interested in understanding how a shrimp’s appendage could smash objects repeatedly without cracking into pieces itself.

The scientists used a variety of techniques to visualize the club. They used light microscopes and electron microscopes to identify key regions of the club. They also used a technique called X-ray diffraction to determine the location of minerals in the club, which make some regions harder than others. After determining the structure, the scientists ran tests on the effect of impact on the club, and developed a computer model describing the transmission of force during an attack with the “biological hammer.”

From this analysis, the researchers learned a great deal about what allows the shrimp’s club to break things without breaking itself. The striking surface of the club contains a mineral (hydroxyapatite) that is found in human teeth, and consequently this region is very hard. But a hard surface alone is likely to crack on impact, especially after repeated use.

In the shrimp’s club, this cracking is minimized by wrapping the minerals with a plastic-like material, thereby keeping any cracks that develop from expanding. Right below this hard surface is an elastic region. This elastic region “bounces” some of the force back, reducing the effect of an impact on the club, and preventing any cracks from enlarging and moving downward.

Based on an understanding of how the club can deliver a pounding and not break itself, scientists think they can develop, among other things, improved body armor.

This research also shows that our fellow creatures can still teach us a great deal about how to build tough, durable materials.

More information is available in an article by James Weaver and others “The Stomatopod Dactyl Club: A Formidable Damage-Tolerant Biological Hammer.” Science 336: 1275-1280, June 8, 2012.

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PATRICK GUILFOILE has a Ph.D. in bacteriology and is currently an interim associate vice president at Bemidji State University.

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