Mantis shrimp floats like a butterfly, hits like a heavyweight
US military research discovers why a predatory shrimp doesn’t destroy itself every time it pounces. Jeff Glorfeld reports.
In the world of boxing, the former heavyweight champion Mike Tyson has been touted as the hardest hitter; and amateur fighter Keith Liddell* holds the Guinness world record for the fastest punch, clocked at more than 70 km/h.
In the animal kingdom both of these destructive features are found in one creature: the mantis shrimp.
Boxers wrap their hands in cloth inside their gloves to prevent injury when hitting their opponents. The mantis shrimp, one of nature's feistiest predators, figured out a similar way to protect the club-like appendage it uses to pulverise prey with incredible speed and force.
In research published this week in the journal Advanced Materials, researchers from the University of California at Riverside, US, have identified a unique structure that wraps around the shrimp's club to protect it from self-inflicted damage as it crushes hard-shelled prey.
Led by UCR’s David Kisailus, the scientists have been studying mantis shrimp in a search for new, ultra-strong composite materials that can be used in aerospace and sports equipment development. The work is funded by the US Air Force Office of Scientific Research.
Mantis shrimp, marine crustaceans of the order Stomatopoda, are aggressive hunters known for killing their prey using a predatory strike that is among the fastest known animal movements. Stomatopods are divided into two groups: "spearers", which attack soft-bodied prey using a harpoon-like structure, and the more recently evolved "smashers", which crush hard-shelled prey using a hammer-like appendage called a dactyl club.
The researchers uncovered how the mantis shrimp uses its smasher to carry out such rapid underwater attacks, which can occur at speeds of up to 23 meters per second. The profile of the club, together with an adjoining region called the propodus, is a hydrodynamic teardrop design that reduces drag resistance.
This teardrop design allows the club to accelerate so quickly that it shears the water, creating imploding bubbles – an effect known as cavitation – to yield a secondary impact on prey.
“Aerodynamic cycling helmets and golf clubs already incorporate this design, suggesting that nature was one step ahead of humans in achieving high-performance structures,” Kisailus says. “The natural world can provide many more design cues that will enable us to develop high-performance synthetic materials.”
In previous research, Kisailus and his team found that the dactyl club is a composite made of mineralised chitin – the same as found in the shells of other crustaceans and insects – but arranged in several unique structures. The exterior of the club, called the impact region, has a hard, crack-resistant coating that enables the mantis shrimp to inflict incredible damage to its prey by transferring its momentum upon impact.
The interior of the club comprises the periodic region, an energy-absorbing structure that dissipates cracks along a series of long, spiral-like fibres; and the striated region.
In the current paper, the researchers show that the striated region has a series of highly aligned fibres that wrap around the club and stop it from expanding upon impact.
“We believe the role of the fibre-reinforced striated region in the smasher's club is much like the hand wrap used by boxers when they fight: to compress the club and prevent catastrophic cracking,” Kisailus says.
“Together, the impact, periodic and striated regions form a club of incredible strength, durability and impact resistance.”
He says a similar striated architectural element is seen in the smasher's more ancient cousin, the spearer, where it is thought to prevent the long, thin barbs from becoming deformed during penetrating strikes.
The study suggests the presence of this structure in the spear likely enabled the advent of the smasher and its biological hammer, a diversification that coincided with the appearance of hard-shelled prey with more sophisticated defences.
The researchers also found a similar structure in the tibia of the land-based praying mantis, suggesting biology has used this design for similar functions.
*The Chicago-born Liddell is also the author of a book, The Tangibility of Nothingness, which theorises the existence of a third number besides zero and one from the binary code.