The mantis shrimp is a colourful, 10-cm-long resident of the ocean whose appearance belies its reputation as one of the most fearsome predators on the planet.
These unassuming crustaceans use a hammer-shaped appendage called the dactyl club to strike their prey at a blistering 23 m/s (about 50-times faster than the blink of an eye), smashing into the poor creature’s body like a bullet from a gun fired point blank. The strike releases enough energy to send small shockwaves through the surrounding water.
But the thing about guns is that every bullet fired has a recoil. It’s Newton’s third law of motion. If a firearm is not securely braced against the body to absorb it, the sudden backward motion can lead to severe injuries.
Yet despite striking prey hard enough to produce shockwaves, the mantis shrimp remains unharmed. How is this possible?
Lasers reveal a shield
A team of researchers from the US and France found the answer in a specialised microstructure in the mantis shrimp’s club. They found that this structure was capable of phononic shielding — a unique ability that allows it to blunt the flow of acoustic waves and thus weaken the recoil the mantis shrimp has.
Their findings were reported in February in Science.
The team fired laser pulses at the microstructure in a rapid sequence that illuminated its response at less than one-billionth of a second at a time. They followed this up with numerical simulations.
“People have looked at the material structure under a microscope but haven’t explored the dynamic mechanical behaviour, especially how it responds to wave propagation,” Maroun Abi Ghanem, the study’s coauthor and a researcher at the Centre National de la Recherche Scientifique, France, said.
“We looked into this behaviour by sending waves through the structure and analysed how they interacted with the material.”
Triggering implosions
The dactyl club of a mantis shrimp stores its energy in spring-like elastic structures held together by latch-like tendons. When the latch is released, the club is released. As it moves to deliver its punch, it displaces the surrounding water and forms small low-pressure zones. Inside these zones, the water’s density drops so much that it turns into vapour, leaving behind a bubble.
When these bubbles collapse due to the pressure of the surrounding water, they release a considerable amount of heat and shockwaves of very high frequencies, up to hundreds of megahertz.
Thus, each dactyl-club punch delivers two blows: one from its own punch and the other from the collapsing bubbles, and together they are capable of breaking the tough shells of clams, mussels, and other crustaceans.
The dactyl club has a hierarchical design — a fine-tuned blend of mineral and organic materials arranged in three layers. The outermost impact surface is made of a thin but hard inorganic material called hydroxyapatite, which distributes the recoil and prevents it from accumulating at one point. Beneath it, the impact layer and the periodic region contain biopolymer fibres arranged in a pattern that can withstand repeated high-intensity impact without incurring significant damage.
Previous studies have explored the club’s material structure and impact resistance. The new study went a step further to check whether the periodic architecture of the materials enhances its mechanical performance.
It does. The team found that the internal arrangement of the microstructure serves as a phononic bandgap: a structure that prevents energy waves of certain frequencies from passing through, or at least significantly attenuated, Horacio Espinosa, a study coauthor and professor of mechanical and biomedical engineering at Northwestern University in Illinois, the US, said.
‘An incredible example’
To mimic the ultrafast club strike in the laboratory, the team used a pair of pulsed lasers that emit very short flashes of light: one to generate energy waves on the material surface and the other to detect them.
When the laser was directed onto a material, it absorbed the light and induced thermoelastic expansion, i.e. heating and expanding the material. This generated a stress wave on the surface, like a miniature earthquake. The team tracked the wave’s movement through the shrimp’s club to understand energy transfer in the material.
The readings helped researchers draw dispersion diagrams — plots that revealed the bandgaps, or specific frequency ranges, where waves couldn’t pass through or were considerably weakened. The appearance of this pattern in the data indicated to the team that the mantis shrimp used phononic shielding to protect itself from the recoil.
“What’s even more fascinating is that our findings suggest the club’s structure not only resists these intense forces but may also be fine-tuned to control how shock waves propagate through it,” Espinosa said. “This dual role of structural robustness and wave manipulation is an incredible example of nature optimising materials at multiple levels.”
Here all along
For a long time, scientists believed that materials that could guide the flow of energy in particular ways could only be created in the lab, not in the wild. Such materials are called metamaterials: they have specially tailored geometries to achieve these effects. The new finding about the mantis shrimp stands to change this belief. Nature always had metamaterials.
The study’s findings can also be applied to develop synthetic sound-filtering materials for use in protective gear, such as earmuffs for soldiers. They could also inspire new approaches to reducing blast-related injuries in the army and sports, the researchers said in a statement.
“We are working on biomimetic structures inspired by the architecture of the mantis shrimp with a focus on wave trapping,” Abi Ghanem said. “We are interested in understanding how the structures trap waves, explore what we can do with this trapped energy, and if it is possible to convert the trapped energy into another form.”
Sanjukta Mondal is a chemist-turned-science-writer with experience in writing popular science articles and scripts for STEM YouTube channels.
Published – April 16, 2025 05:30 am IST
