As ambush predators, mantis shrimp excel at lying in wait for their prey before striking from the shadows with specialised feeding appendages, but the form and function of these appendages has divided mantis shrimp species into two evolutionarily distinct groups. There are ‘spearers’, which use sharp harpoon-like appendages to slash and stab at soft-bodied prey, and ‘smashers’, which use blunt hammer-like appendages to break into hard-shelled crustaceans. These bulbous smashing appendages, known as dactyl clubs, are capable of accelerating at 100,000 m s−2 (similar to a small-calibre bullet) and applying crippling forces equal to 2500 times the body weight of the mantis shrimp. Wielding such powerful weapons makes the mantis shrimp a formidable predator, but how are they able to apply such devastating blows and remain undamaged after constant use?
The answer to this cracking question lies in the complex regional microstructures of the mantis shrimp feeding apparatus. A team led by Lessa Kay Grunenfelder, from the University of California, Riverside, USA, built on previous work investigating the structural regions of these feeding appendages and focused on examining the structure and evolutionary history of the previously unexamined ‘striated region’. Using both optical and electron microscopy, the team carefully examined and compared cross-sections taken from the appendages of the ‘smashing’ peacock mantis shrimp, Odontodactylus scyllarus, and the ‘spearing’ zebra mantis shrimp, Lysiosquillina maculata, in order to learn more about the role of this region.
Their findings, published in Advanced Materials, reveal that the striated region of the smasher O. scyllarus is composed largely of tightly packed sheets of mineralised chitin, compared with the organic components of the surrounding regions. They also found that this region of toughened chitin wraps all around the club, leading the authors to compare its role with that of the hand-wrap used by boxers, condensing the fist into a tight ball and preventing catastrophic injury upon impact. This region was also found to contain a network of pore channels that allow for ion transport and give the region its striated appearance, but may also hold the potential for self-healing following particularly devastating impacts.
Interestingly, when the team looked at the appendages of the spearer, L. maculata, they found a similar striated region, even though this species is subjected to very different pressures during a feeding strike. In contrast, the fibres in the spear are aligned in a position more suited for longer appendages, which may help to reduce deformation of the stabbing appendage when connecting with prey. As the spearers evolved prior to the appearance of the smashers, the team believe that this microstructural feature may have been instrumental in the evolution of the first club-wielding mantis shrimps as a response to the appearance of hard-shelled crustaceans. Finally, the team identified the characteristics of a similar striated structure in the limbs of a praying mantis, Stagomantis limbata, revealing that terrestrial arthropods may also have convergently evolved similar structures for their own hunting needs.
With the aim of applying this knowledge for humanity's benefit, the team go on to describe how the microstructures of these weaponised feeding appendages could be used to influence the biomimetic design of new materials. The authors highlight their particular interest in the aerospace and sports industries with the goal of creating more durable aerofoils for aircraft, as well as bicycle helmets and golf clubs that maximise speed and strength without sacrificing structural safety. In these turbulent times, it is refreshing to see that scientists are hoping to turn one of nature's deadliest and most durable ‘swords’ into ploughshares.
