5 minute read
BUILT FOR SPEED
While mantis shrimps typically grow to around 10cm, they boast possibly the fastest hunting strike in the animal kingdom. Naturalist Georgina Jones reveals more about these fascinating creatures that have also mastered the art of deception quite well.
Though they spend the bulk of their lives in burrows or caves, when it comes to hunting, mantis shrimps are all about speed. And their bodies are superbly adapted to this purpose.
Mantis shrimps are named for the resemblance their striking limbs have to those of praying mantises. The family first diverged from the rest of the crustaceans nearly 400 million years ago, and the ancestors of all modern species are recognisable in the fossil record from nearly 200 million years ago. There are about 450 known species, generally divided into spearers and smashers.
Spearing mantis shrimps hunt by impaling their prey on their raptorial claws and smashers use a hardened club to smash their prey. Researchers have found their strikes range in speed from 10 to 23m/s (36-83km/h) – probably the fastest hunting strike in the animal kingdom.
Studies on the hunting limbs suggested a power amplification mechanism was being used: the limbs have a muscle that contracts slowly over time and then is latched once contracted so that when it unlatches, the stored energy is released rapidly, contributing to the strike speed.
So fast is the strike, in fact, that mantis shrimps also use the extremes of fluid dynamics to overwhelm their prey. Cavitation damage is well known to mariners. It is caused when areas of water close to each other move at radically different speeds and generate areas of such low pressure that the water turns to gas and forms a short-lived bubble. When the bubble collapses, it emits sound, light and heat – the heat being close to the temperature of the surface of the sun for a brief instant – and the cavitation bubble produced along with the mantis shrimps’ strike is what does the most damage to their prey.
It also damages the mantis shrimps. Though they have thick layers of chitin and nanocrystal shock absorbers below their exoskeletons, the hunting clubs of smashers have been observed worn right through to the flesh. They solve this problem every time they moult, when they replace their entire exoskeletons.
In order to strike so fast, however, they need to see really well. And in fact, mantis shrimps have the most complex eyes known. Each eye is mounted on a stalk and is capable of independent movement. These are compound eyes and each is divided into three parts, giving mantis shrimps effective trinocular vision in each eye, so that each eye is capable of depth perception. In addition to this, mantis shrimps have between 12 and 16 photoreceptors. They can see wavelengths from the deep ultraviolet to far into the red spectrum as well as polarised light. Compared to human beings, with only four photoceptors, this seems as though mantis shrimps must see amazingly more than we do. But mantis shrimp vision is all about speed. So where human beings have many colour-specific neurons, mantis shrimps rather have the image processing happen in their eyes, so that the signal to their brains can be processed for action really quickly, rather than requiring a larger brain for image processing.
It is thought that their extreme variety of photoreceptors allows them to detect different types of coral, transparent or UV-emitting prey species, or predators. Also, their pair of depth-perceiving eyes gives them instant accurate information on where to aim their strikes at speed.
Other parts of their visual systems aren’t for speed: mantis shrimps fluoresce during mating rituals, and female fertility is linked to the lunar cycle, so mantis shrimp eyes of course are adapted to detect both fluorescence and the phase of the moon.
Mantis shrimps can live for up to 20 years and can have up to 30 breeding cycles. Some species will lay their eggs in a burrow while others carry the eggs around with them until they hatch. In some cases, the female will lay two sets of eggs, one for each parent to tend. In others, there is egg care by both parents, or the female cares for the eggs while the male hunts for her food. After hatching, the larvae may spend up to three months in the open ocean before settling out onto the reef or sand that will be their adult home.
As they grow, they grow out of their exoskeletons, and must moult. Useful, as mentioned earlier, for replacing hunting damage. But during moulting they are unable to strike. So mantis shrimps bluff. Their behaviour becomes increasingly aggressive pre-moult, and they make sure to strike at anything and everything that comes near their burrows. During moult, if not safely hidden, they continue to wave their striking limbs around. With luck, enough predators have been effectively intimidated by their previous aggressive behaviour to allow them to complete their moults and continue their high-speed hunting lives.
