Mantis shrimps pack a mighty punch. An average sized shrimp can hurl a claw at speeds up to 23 m s–1 with forces up to 1500 N – thousands of times its own body weight – and the whole incident is over in the blink of an eye. ‘These animals are unbelievably fast,’ says Jennifer Taylor from the University of California, Berkeley. But mantis shrimps don't just use their claws to pulverise passing prey. They indulge in ritualised duels over burrows; taking it in turns to thump each other on the tail (telson) without injury until one of the contestants backs down. Realising that the telson must be incredibly robust to take such a pounding, Taylor and Sheila Patek decided to look at its responses to high impact collisions to find out how the telson emerges unscathed from duels (p. 3496).
Simulating the impact of a claw by dropping a small steel ball on the central ridge segment (carina) of a telson, Taylor filmed the impact at a staggering 15,000 frames s–1. ‘The telson from this stomatopod feels so stiff you wouldn't expect to see any deformation but you actually see it moving in the video,’ says Taylor. And when she looked closer, she realised that instead of the site of impact deforming, the entire domed section (composed of three carinae) depressed. ‘The cuticle in between the carinae was more flexible and allowed for the deformation that I saw,’ explains Taylor.
Curious to find how the telson dissipates collision energy without shattering, Taylor measured the telson's coefficient of restitution (e) – the ratio of the separation velocity to the impact velocity (ranging from 0 to 1), used by engineers and sports scientists to analyse impacts – to find out if the telson behaves like a rebounding trampoline (high e) or an energy absorbing punch bag (low e). Carefully measuring the velocity of the ball over the final 10 frames of the descent and the first 10 frames after the impact, Taylor found that its coefficient of restitution was 0.56: the ball lost 69% of its energy in the impact. The telson was behaving more like a punch bag or stiff spring than a trampoline. And when she measured the duration of the impact and impulse (change in ball momentum) as the ball smashed into different sized telsons, Taylor realised that the larger the telson the longer the impact and the smaller the impulse, information that mantis shrimps could use to tell them about their opponent's size.
Finally, having measured the telson's responses to high impact collisions, the duo decided to look at the distribution of minerals in the telson to see if they could find any patterns to explain its resilience. Patek CT scanned the telson and found that all three carinae were highly mineralised, while the surrounding regions had relatively low mineralisation. ‘It appears that the carinae provide stiffness, while the cuticle surrounding them provides compliance,’ say Taylor and Patek, who point out that man-made impact resistant armour, composed of stiff regions interspersed with flexible sections, uses exactly the same strategy to protect humans from impact injuries.
Having discovered how mantis shrimp telsons withstand blows that would shatter other crustacean's shells, Taylor is keen to find out whether telsons have adapted to withstand these forceful impacts, but this will have to wait until she sets up her own lab later this year.