Mantis shrimps are famous for two things: perhaps the most extraordinary visual capacity of any animal, and the most powerful punch in the animal kingdom. Now a new twist has been placed on the second, with the discovery that mantis shrimps can only perform their extraordinary jabs underwater. In air, they limit themselves, probably lest they injure themselves.
Mantis shrimps kill prey and fight off enemies using limbs known as maxillipeds that punch or spear targets. The speed of movement of these limbs is the fastest ever recorded for an animal. One mantis shrimp reportedly shattered the bulletproof glass of the aquarium in which he was housed.
The mantis shrimp punch is even more extraordinary when you consider it is conducted underwater, where viscous drag slows movements down. Dr Kathryn Feller of the University of Minnesota wondered how fast mantis shrimps would manage if this restriction was removed. Because scientists get to be cool sometimes she tested it by restraining seven shrimps of the species Squilla mantis in place and tickling their abdomens to induce punching. Because science can also be dangerous, Feller reported: “I have a pretty epic photo of my bleeding hand over a white sink when one stabbed me during this process,” in a background piece.
The aquarium's water level was adjusted so the punches were thrown sometimes underwater and sometimes in air, but the subjects' gills were always safely beneath the waterline. Movements were filmed with 5,000-frames-per-second video cameras.
To her surprise, Feller found instead of relishing their new-found freedom, these tiny pugilists instead pulled their punches, moving at not much more than half the speed and unleashing a third the kinetic energy. On investigation, she concluded the shrimps deliberately limit their strike force in air.
Mantis shrimps achieve their remarkable punching power not with muscles, but with what are known as “spring-actuated systems”. Locusts and fleas use something very similar to leap into the air. Like an archer storing energy in a bow to be released very suddenly, the mantis shrimp's maxillipeds are latched into place before it uses its muscles to load energy in a spring, which then releases it far more rapidly than it was stored.
Given that more powerful punches underwater don't seem to tire them out much, Feller thinks this modulation must reflect a risk that, if not dissipated by the water's drag, the mighty punch could harm the puncher as well as the target, perhaps through lack of a suitable braking mechanism. Locusts have a shock-absorbing material in their legs to prevent damage from excess kinetic energy; mantis shrimps are not known to have anything equivalent, so may have worked out long ago not to push too hard in air. “The animals don't give it the full pow so they don't blow out their joint,” Feller speculated.
Feller notes in the Journal of Experimental Biology: “S. mantis strikes in air have a similar mass and performance to latch-mediated spring-actuated jumps in locusts, suggesting a potential threshold for the energetics of a 1-2 gram limb rotating in air.” In other words, mantis shrimps may be operating at the limits of what is biologically possible.
One disadvantage of spring-actuated systems is they happen so fast no animal's brains can stop, or even alter the movement, once it is engaged.