How A Peacock Shrimp Packs A Punch

Anybody who has repeatedly punched a wall knows that one’s fist typically suffers as much damage as it inflicts. In contrast, four-inch-long peacock shrimp living in the Pacific and Indian Oceans repeatedly smash through the shells of unsuspecting prey without damaging their own pretty red clubs. That capacity is due to unique layering of stiff and compliant materials in the animal’s club, researchers report in Science (DOI: 10.1126/science.1218764). The club’s chemical makeup could provide a blueprint from which to design resilient materials for body armor and shields.

Less than a quarter-inch long, the shrimp’s club strikes with 200 lb of force, enough to break a glass aquarium, says David Kisailus, a chemical engineer at the University of California, Riverside. “Its club accelerates faster than a .22-caliber bullet, and all this happens underwater,” he adds. “It can also strike thousands of times without breaking.”

Kisailus and his colleagues used scanning electron microscopy, X-ray diffraction, micromechanical testing, and computer micromechanical modeling to establish that the club’s strength is a result of the cooperation of three layers of materials.

The impact surface of the club is made up of extremely dense hydroxyapatite. The compressive strength of this region is greater than that of high-temperature engineering ceramics such as zirconium oxide and silicon carbide, Kisailus says. Below the hydroxyapatite is a more compliant layer composed of chitin fibers arranged in helical spirals and surrounded by amorphous mineral. Finally, more chitin wraps around the edges of the club. Unlike the helical chitin fibers in the second layer, these chitin fibers are parallel to each other. The arrangements of both chitin fiber layers, as well as the interface between the hydroxyapatite and chitin regions, prevent major cracks.

“Modern body armor also makes use of a layered structure . . . . However, the ceramic plates [used] fracture on impact and have to be replaced,” says K. Elizabeth Tanner, an engineer at the University of Glasgow, in Scotland, in an associated commentary in Science. Engineers may wish to look to the shrimp’s club to improve the impact resistance of shields over multiple blows, she adds.