The mantis shrimp is famous in the animal kingdom for its fast, powerful hammer strike, on par with the force generated by a .22 caliber bullet. One might conclude that those strikes would be even faster and more formidable in air, given the lower density and less drag of the medium. But that's not the case, according to a recent paper in the Journal of Experimental Biology. Rather, scientists found that the animal punches at half the speed in air, suggesting that the mantis shrimp can precisely control its striking behavior, depending on the surrounding medium.

Mantis shrimp come in many different varieties: there are some 450 known species. But they can generally be grouped into two types: those that stab their prey with spear-like appendages ("spearers") and those that smash their prey ("smashers") with large, rounded, and hammer-like claws ("raptorial appendages"). Those strikes are so fast—as much as 23 meters per second, or 51mph—and powerful, they often produce cavitation bubbles in the water, creating a shock wave that can serve as a follow-up strike, stunning and sometimes killing the prey. Sometimes a strike can even produce sonoluminescence, whereby the cavitation bubbles produce a brief flash of light as they collapse.

Per a 2018 study, the secret to that powerful punch seems to arise not from bulky muscles but from the spring-loaded anatomical structure of the shrimp's arms, akin to a bow and arrow. The shrimp's muscles pull on a saddle-shaped structure in the arm, causing it to bend and store potential energy, which is released with the swinging of the club-like claw.

Kate Feller, a co-author of the recent study who is now at the University of Minnesota, had been conducting a physiological study of mantis shrimp in the lab while still at the University of Cambridge in the UK. The creatures really don't like those controlled conditions and tend to lash out, especially when exposed to air. Feller figured out how to hold the shrimp in such a way that their gills remained under water, even though the appendages they use to strike were exposed to air. Her Cambridge colleague and co-author, Greg Sutton, visited her lab one day and mentioned in passing that it might be interesting to measure the force of the shrimps' hammer blows in air. And thus this latest study was born.

Feller and her collaborators experimented with six females and one male mantis shrimp. To control for any variation in body posture, each shrimp was partially restrained on a gimbaled platform in an aquarium partially filled with seawater. Thus mounted, the animals were placed in the aquarium, sometimes fully submerged, sometimes partially.

The scientists then gently poked each shrimp in the posterior with a fiberglass stick to get it to strike out defensively, all while using high-speed video to capture the movement. And no, the shrimp did not appreciate being poked. "I have a pretty epic photo of my bleeding hand over a white sink when one stabbed me during this process," said Feller.

All told, the team analyzed 31 strikes in the air and 36 strikes in the water. Feller had expected to find that the blows would be as powerful, if not more so, in the air, but the analysis showed just the opposite. The strikes were half as fast, averaging roughly 5 meters per second, or 11mph. In fact, Feller et al. noted in their paper that the kinetic energy output of the mantis shrimp in air is similar to that of a grasshopper's leg, while the shrimp could achieve 10 times the power when striking in water.

Our little kablammo talk

Why might this be the case? Feller et al. think it might be related to the need for some kind of shock-absorbing capability, pointing out that the limbs of locusts and similar leaping insects have built-in structures to absorb excess kinetic energy. A 2012 study found that the mantis-shrimp claw is also good at absorbing energy, thanks to an inner layer made of chitin (commonly found in the shells of crustaceans), calcium phosphate (found in human bone), and calcium carbonate. A 2016 paper from the same group found that the claw's outer layer also contained chitin fibers, surrounded by calcium phosphate arranged in a precise herringbone pattern.

Granted, some of the excess energy is also delivered into the target—like hard snail shells, for instance. But this latest study indicates that the medium might contribute to shock absorption as well. "The surrounding medium and strike target work together as external shock absorbers for mantis shrimp strikes," the authors wrote.

In other words, perhaps water is better at dissipating the excess kinetic energy produced by the strikes, so the shrimp don't have to pull their punches quite as much as they do when striking in air. "In air, not only are the forces of drag from water absent, but the entire sensory experience is messed up," said Feller. "So maybe—in the absence of a perceived target—the animals don't give it the full pow so they don't blow out their joint."

DOI: Journal of Experimental Biology, 2020. 10.1242/jeb.208678 (About DOIs).