Mantis shrimp are known for packing a powerful punch. With the force of a .22 caliber bullet, these colourful invertebrates can smash shells without hurting themselves. Curious to better understand this pelagic pugilist, researchers from Northwestern University in Illinois believe they’ve worked out just how these fascinating creatures can withstand the intense shockwave created by their own deadly strike.
“The mantis shrimp is known for its incredibly powerful strike, which can break mollusk shells and even crack aquarium glass,” said Northwestern’s Horacio D. Espinosa, the study’s co-corresponding author and an expert on bio-inspired material, who co-led the study.
Of course, the answer to how they can withstand the blow is rooted in some deeply fascinating science that may just subvert some expectations.
“To repeatedly execute these high-impact strikes, the mantis shrimp’s fists – or their dactyl clubs – must have a robust protection mechanism to prevent self-damage,” explained Espinosa. Most prior work has focused on the club’s toughness and crack resistance, treating the structure as a toughened impact shield.”
But this wasn’t the answer. Rather, scientists have found, it is the mantis shrimp’s ability to filter out sound.
“We found it uses phononic mechanisms – structures that selectively filter stress waves,” said Espinosa. “This enables the shrimp to preserve its striking ability over multiple impacts and prevent soft tissue damage.”
What this means, is that the mantis shrimp’s dactyl clubs are, in fact, covered in specially evolved layered patterns which selectively filter out sound. By blocking specific vibrations, these patterns then act like a shield against self-generated shockwaves.
Mantis shrimp – of which there are around 400 different species – live in shallow, tropical waters in the Pacific and Indian oceans where they reach lengths of around 10cm. They have rainbow coloured shells and large, purple eyes that sit on top of stalks which they can move independently. Despite their name, mantis shrimp are not, in fact, shrimp, but a stomatopod type which are relatives of crabs and lobsters.
These impressive animals are armed with one hammer-like dactyl club on each side of its body. These clubs store energy in elastic, spring-like structures, which are held in place by latch-like tendons. When the latch is released, the stored energy, too, is released — propelling the club forward with explosive force.
With a single blow, mantis shrimp can kill their prey or defend their territory from interloping competitors. As the punch rips through surrounding water, it creates a low-pressure zone behind it, causing a bubble to form.
“When the mantis shrimp strikes, the impact generates pressure waves onto its target,” Espinosa said. “It also creates bubbles, which rapidly collapse to produce shockwaves in the megahertz range. The collapse of these bubbles releases intense bursts of energy, which travel through the shrimp’s club. This secondary shockwave effect, along with the initial impact force, makes the mantis shrimp’s strike even more devastating.”
But, to the surprise of the researchers, this extensive force does not damage the shrimp’s delicate nerves and tissues, which are encased within its armour.
To investigate this phenomenon, Espinosa and his colleagues – including M. Abi Ghanem of the Institute of Light and Matter, a joint research unit between Claude-Bernard-Lyon-I University and France’s Center for National Scientific Research, with whom he carried out the study – used two techniques: One laser-based method to analyse how stress waves propagate through materials; the other using laser ultrasonics which provided further insights into the armour’s microstructure, allowing researchers to examine the mantis shrimp’s armour in finer detail.
