Animal kingdom's most powerful puncher generates a 'phononic shield' to protect itself

Mantis shrimps pack a powerful punch — and scientists have finally figured out how this super-strong strike doesn't obliterate the shrimps themselves as they lash out. Turns out, these shrimp have a special shock-absorbing "shield" to help them survive as they deliver shell-crushing blows.

The punch of a peacock mantis shrimp (Odontodactylus scyllarus) is the strongest self-powered strike by an animal. They use hammer-like fists, or dactyl clubs, to shatter prey's shells. The strike is so strong it can even break aquarium glass, delivering a force comparable to a .22 caliber bullet.

But because these high-impact strikes generate a lot of force, scientists have puzzled over how the critters can withstand the intense shock waves generated by their own attack.

In a new study published Feb. 6 in the journal Science, researchers examined the structure of the shrimps' clubs. Their findings revealed that the microstructure of these clubs act as natural shock absorbers to limit damage.

"We found it uses phononic mechanisms — structures that selectively filter stress waves," study co-author Horacio Dante Espinosa, a professor of mechanical engineering and biomedical engineering at Northwestern University, said in a statement. "This enables the shrimp to preserve its striking ability over multiple impacts and prevent soft tissue damage."

Powerful punch

Peacock mantis shrimp use a complex system of biological latches and springs in their dactyl clubs to unleash a punch at a speed of 75 feet per second (23 meters per second), according to a 2004 study — 50 times faster than the blink of an eye.

While this immense speed helps deliver a powerful blow, it also creates dangerous shock waves.

"The strike is so fast that it creates cavitation bubbles, which, upon collapsing, generate additional shockwaves, effectively delivering a double impact," Espinosa said.

Previous research theorized that the microstructure of the dactyl clubs helps protect the shrimps from these shock waves.

In the new study, the scientists tested this theory using advanced laser-based techniques to analyze how different wavelengths move through the peacock mantis shrimp's dactyl clubs.

The findings revealed two important regions in these clubs that help them survive their own strikes: the impact region and the periodic region.

The impact region is composed of a layer of chitin fibers arranged in a herringbone pattern that reinforces the club against fractures.

Beneath this layer is the periodic region, made from twisted arrangements of layered chitin fibers. This type of helicoidal structure is known as a Bouligand structure and is found in fish scales and lobster exoskeletons to provide strength and fracture toughness.

The laser tests measured the speed of acoustic stress waves through both regions. These waves passed through the impact region unchanged but moved at varying speeds through the periodic region — suggesting the latter region causes high-frequency waves to disperse to reduce the intensity.

The researchers also discovered that the periodic region filtered out high-frequency shock waves — which can cause significant damage to tissues, according to the statement.

The high-frequency waves were likely generated when the cavitation bubbles collapsed.

"We connected this high frequency to the frequency generated by bubble collapse during the impact event," Epinosa said.

The bundles of fibers in the periodic region act like a "phononic shield," actively blocking, redirecting and scattering waves, and ultimately preventing any harmful shock waves from traveling efficiently through the layer. This protects the delicate tissues of the mantis shrimp from the resulting shock waves of the cavitation bubble.

"The research provided experimental evidence that the Bouligand structure of the mantis shrimp's dactyl club functions as a phononic shield, selectively filtering high-frequency shear waves generated during impact," Espinosa said.

"These features help protect the mantis shrimp's club from damage by mitigating high-frequency stress waves, making it a naturally optimized impact-resistant structure," Epinosa said.

According to the press release, this study could be applied to the development of sound-filtering materials for protective gear and inspire new approaches to reduce blast-related injuries in the military and high-impact sports.