High impact science

The exoskeletons of arthropods allow them to do many seemingly preternatural things. Storing energy to generate the fastest limb movements in the animal kingdom is one of those things. Stomatopods, or mantis shrimp, aren't the fastest, but the hero of this story (Odontodactylus scyllarus) can accelerate its killing arms incredibly fast (65 - 104 km s^-2) reaching top speeds of 23 meters per second. To my knowledge, that's the second fastest limb movement ever recorded. And it's performed in water, which strongly limits speed relative to air.

The stomatopod Odontodactylus scyllarus, or peacock mantis shrimp (photo Wikipedia)

Stomatopods are coarsely divided into two groups based on the shape of their killing arms. Some stomatopods have arms that look similar to those of praying mantises, which is where their common name is derived. These stomatopods are called "spearers", as they use their arms to impale soft-bodied prey. The other group, which use club-like arms to break open the shells of other crustaceans and mollusks, are called "smashers". But, there is a great diversity in the form and function of stomatopod arms. The largest stomatopods tend to be spearers, but the fastest are smashers.

A sample of the diverse shapes of stomatopod killing arms (photo Thomas Claverie)

Our speedy pal, O. scyllarus, is a relatively large smasher, growing to 18 centimeters. Although water does hamper the speed of its strike, it also provides an interesting advantage. The fast movement of the club-like killing arm causes a cavitation bubble to form between the club and the point of impact. Cavitation bubbles form in areas where water pressure drops so low it forces a phase change from liquid to vapour. The higher pressure in the surrounding water then causes the vapour bubble to collapse rapidly, releasing a shock wave of sound and a burst of light. The collapse of the cavitation bubble is so violent that it can impart a force as great as the club strike on the prey item.

 

The stomatopod O. scyllarus in action

The double strike of the club and cavitation bubble causes an impressive amount of damage to the hard-shelled prey of O. scyllarus. But, the impact of the strike and cavitation bubble also cause damage to the club itself. The only time this damage can be repaired is when the stomatopod moults. Although O. scyllarus does moult relatively frequently, its clubs are still resilient enough to deliver thousands of blows between moults.

The resilience of the club's hitting surface is due to a complex, three-region architecture that allows small cracks to form, but prevents them becoming large enough to be a problem. The outer 'impact region', which forms the hitting surface, contains a highly crystalised form of the mineral hydroxyapatite. The inner 'periodic region' also contains hydroxyapatite, but here it occurs in the amorphous mineral phase and is interspersed with the carbohydrate chitin, which is arranged in helical stacks. The 'striated region', which forms the back and sides of the club, is made of chitin arranged in circumfrential bands.

Cross sections of an O. scyllarus club, illustrating the three-region architecture. The crystalised hydroxyapatite impact region, the amorphous hydroxyapatite and chitin periodic region and the chintinous striated region. The lower panel shows the distribution withing the club; blue for the impact region, red and yellow for the periodic region and green for the striated. The orange indicates another segment of the killing arm that sits behind the club and acts as the 'handle' (image modified from Weaver et al. 2012).

The periodic region is the 'shock-absorber' of the system and the majority of cracks that form in the club occur in this region. The cracks that form are forced to twist inwards by the helically arranged chitin and are largely prevented from spreading between layers or into the impact region. The striated region of chitin reduces the deformation of the club during strikes and helps to keep the cracks contained within the periodic region. This unique architecture allows the club to survive the damaging forces of many high speed strikes on hard-shelled prey.

If you would like to hear more about the fast strike of O. scyllarus, including high speed footage of the cavitation bubble and images of damage to the club, I recommend you check out Shelia Patek's TED talk.

References:

Patek, S., Korff, W., & Caldwell, R. (2004). Biomechanics: Deadly strike mechanism of a mantis shrimp Nature, 428 (6985), 819-820 DOI: 10.1038/428819a

Weaver, J., Milliron, G., Miserez, A., Evans-Lutterodt, K., Herrera, S., Gallana, I., Mershon, W., Swanson, B., Zavattieri, P., DiMasi, E., & Kisailus, D. (2012). The Stomatopod Dactyl Club: A Formidable Damage-Tolerant Biological Hammer Science, 336 (6086), 1275-1280 DOI: 10.1126/science.1218764