How is a mantis shrimp like a punching bag? The way it takes a hit.

Mantis shrimp are, ounce for ounce, some of the most fearsome predators that you can pull out of the ocean. The marine crustaceans of the order Stomatopoda (neither shrimp nor mantids, they got the name because of their physical resemblance to both) are tiny and unassuming, but can use their front claws to attack with incredible speed and tremendous force. Stomatopods armed with “smasher” claws (there are also those armed with spearing claws) regularly crack open crabs and snails with cudgels that work on the same principal as crossbows: a spring-and-catch mechanism allows potential energy to be built up and stored and then released all at once. When all that power is unleashed, stomatopods can bludgeon prey with 45 mph strikes (the fastest known limb movement in the animal kingdom) and 340 pounds of force.

These war hammers aren’t just for hunting meals, though. Stomatopods use them on each other in territorial disputes, too. Given what these strikes can do, how have mantis shrimp not power-punched each other into extinction?

There’s two parts to the answer. One is the way they hit each other. When sparring over turf, two mantis shrimp will usually exchange a few strikes to each others’ tails as a way of sizing each other up before committing to a full-on and rumble and mutually assured destruction. The second part is where they hit each other. These ritual test blows are made to each other’s telsons, armored tail segments that are strong enough to take the punishment.

To find out just how strong telsons are and how they withstand such force, Sheila Patek from the University of Massachusetts and Jennifer Taylor, from the University of Indiana took a few shots of their own at them. They got some mantis shrimp, let them live out a few final days eating grass shrimp in luxurious plastic cup accommodations and then put them in the freezer until they were dead, but not frozen solid. Then, they superglued the stomatopods to a strip of Plexiglass and dropped stainless steel balls on them.

The pair recorded the impacts with high-speed video cameras and used the data to calculate the tails’ coefficient of restitution, a value representing the elasticity of an object. The basic principle of the measurement is that the amount of elastic energy absorbed by an object can be measured by the loss of momentum of a colliding object suffers, so the figure is expressed as a ratio of and the post- and pre-impact velocities of the striking object. Coefficients of restitution are often used to characterize and regulate products that take their fair share of blows, like automobiles, body armor, sports equipment and even fruits and vegetables.

Patek and Taylor calculated the telson’s coefficient of restitution as 0.56. This is similar to a major league baseball, which has a coefficient of restitution between 0.45 and 0.50 when hit with a bat. The telson dissipated a significant amount of energy, 69%, when it compressed during impact with the steel balls. The incredible loss of energy implies that the telson absorbs impact inelastically, like a heavy punching bag does.

Patek and Taylor also used micro-ComputedTomography (a 3-D imaging method that uses penetrating waves) scans to examine mantis shrimp exoskeletons to see if they could find anything that might explain the telson’s resilience.

They found that the stomatopods’ tails are two times thicker than normal at three ridges, called carinae, that run along the telson. While the center area of the telson crumples inward upon impact, the carinae don’t deform. This provides a balance of stiffness and compliance that helps impact resistance by both absorbing energy and resisting penetration, a strategy human engineers have co-opted for designing armor.

Reference: Taylor JR, & Patek SN (2010). Ritualized fighting and biological armor: the impact mechanics of the mantis shrimp’s telson. The Journal of experimental biology, 213 (Pt 20), 3496-504 PMID: 20889830

Patek, S., Korff, W., & Caldwell, R. (2004). Biomechanics: Deadly strike mechanism of a mantis shrimp Nature, 428 (6985), 819-820 DOI: 10.1038/428819a

Images: Female Odontodactylus Scyllarus by Roy L. Caldwell, UC Berkeley, for the National Science Foundation. Odontodactylus Scyllarus by Flickr user prilfish, used under a Creative Commons license.


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