Two recent articles in the most recent issue of Journal of Experimental Biology (JEB) featured animals that possess impressive “ballistic movements”, which sparked my interest in the mechanics of superfast motion. This is the first in a series of blog posts featuring animals that are capable of performing incredible high-velocity movements.
What Are Ballistic Movements?
Movements that are deemed ballistic require very high velocity muscle contractions that are usually quite brief but can result in high force production and huge power outputs. Ballistic movements often require the co-activation of multiple counteracting muscles, agonists and antagonists, to produce the coordinated and stable movement of a bodily appendage at a rapid speed. The activation of these muscles initiates contractions that generate mechanical work, which is stored as potential energy in elastic mechanisms such as tendons. When triggered, the stored energy is released in a very brief duration that results in a superfast spring-released movement. The fastest of these movements are observed in predatory organisms that rely on speed to catch fast-moving prey.
The Killer Shrimps
Mantis shrimps (crustaceans of the order Stomatopoda) are one such group of organisms! There are many fascinating aspects of mantis shrimps, including their eyes and ritualised fighting behaviours, but for this post we’re focusing on their speedy biological weapons.
Mantis shrimps have adapted specialised front claws for hunting and feeding, known as raptorial appendages, that can be distinctly grouped into ‘smashers’ and ‘spearers’. Smashers have developed their appendages into round clubs that are used to smash through the shells of other crustaceans, and spearers have developed elongated sharp claws that are used to harpoon softer-bodied prey from a distance and drag them closer towards the awaiting shrimp.
These strikes, particularly those performed by smashers, are incredibly fast and capable of dealing damage that has been known to crack aquarium glass. The strikes of smashing mantis shrimps can accelerate at 100,000 m/s/s (similar to that of a small-caliber bullet) and reach speeds of 100 km/hr. The strikes themselves are so brief (1-6 ms) that 100 strikes can occur in the time of one human eye blink, delivering incredibly high forces during this time. Below are videos of both smashers and spearers in action.
In fact, these strikes are so fast that cavitation bubbles, or vapour pockets, are generated. When these bubbles collapse, they send a shockwave of force outwards – essentially dealing a second blow to the target! These collapsing bubbles can also produce a small amount of light by sonoluminescence as the temperature of the vapour in the bubble is rapidly increased, visible in the video below. The temperature of the cavitation bubbles created by snapping shrimps, cousins of the mantis shrimps, has been estimated to reach 4,700 °C, and for comparison, the surface of the sun is estimated to be approximately 5,500°C!
Staying In Control
In this month’s JEB, Dr Katushi Kagaya and Dr Sheila Patek present evidence that the Caribbean rock mantis shrimp (Neogonodactylus bredini) has a neurological system in place that allows them to adjust the timing of their ‘smashing’ strikes after the muscles have finished contracting.
The team measured muscle activation patterns with electromyograms of the extensor and flexor muscles responsible for the spring compression and latch release of the raptorial appendages over a series of feeding strikes. They found that the initiation of the strikes following the co-contraction of the extensors and flexors is likely due to the release of the flexor controlling the latch mechanism, which may mean that the precise timing of the strike is determined by the flexor activity.
They also found a degree of variation in the relative timings of the activation periods and pre-release delays between repeated strikes from individual shrimp and between multiple shimp. This may suggest that these shrimps are able to acutely modify the delay of the latch release mechanism and alter the timing of the strikes to best suit the moment. These timing modifications appear to be triggered through the central nervous system (CNS), suggesting these animals are capable of rapid and precise neurological control of the strike release.
These results open up many more questions about the cognitive faculties of these creatures, especially regarding prey assessments and adaptive learning. The team also question whether the these minute changes in timing are in response to sensory signals received from mechanosensory structures or if there is some interneuronal activity that triggers the release, something that future work may shed some light on.
Top image credit: Roy Caldwell / Wired