But what exactly are mantis shrimps using their ridiculously elaborate eyes for?
Animals do the most amazing things. Read about them in this series by Janaki Lenin.
We have three colour receptors – red, green and blue. Birds have these three receptors as well as a fourth to detect ultraviolet. But mantis shrimps have 12 to 16 colour receptors, an order of magnitude unknown in any other creature. This doesn’t mean these marine crustaceans see an unimaginable array of colours invisible to the human eye. Their colour vision is, in fact, poorer than ours, but their eyesight is extraordinary in other ways. For instance, while we need two eyes to perceive depth, mantis shrimps do it with one of theirs.
Light travels in waves, vibrating in every direction. It gets polarised when it beams through a polarising filter or if it bounces off glass or water. This causes it to pulsate in one direction only, a phenomenon called linearly polarised light. Light can also spiral clockwise or anti-clockwise to create circularly polarising light. Filmmakers use this phenomenon to create 3D movies.
Not only do mantis shrimps see the visible spectrum like us, they can see ultraviolet light like birds, and linearly polarised light like insects and octopuses. They can also see circularly polarised light that no other animal can. Now researchers have uncovered how the crustaceans that look like colourful lobsters see polarised light – it’s a simple yet strange technique called ‘rolling the eyes’.
The compound eyes of mantis shrimps sit atop a pair of stalks. They move in three ways – up-down, side-to-side, and roll – either together or independently. A mid-band made up of six rows of special cells separates each eyeball into two hemispheres. Two of these rows detect polarised light. This narrow band has to move to register colour in the seascape. So mantis shrimps perform “frequent, small, and relatively slow movements, which give the animal a strange inquisitive appearance,” wrote Professor Emeritus Michael Land of University of Sussex, UK, in 1999.
Nicholas Roberts of the University of Bristol, UK, and his team investigated why mantis shrimps rolled their eyes. “We have known for a while that mantis shrimps see the world very differently from humans,” Roberts said in a press release. “The eye movements of mantis shrimps have always been something of a puzzle.”
The scientists caught purple spot mantis shrimps from Lizard Island, Australia, and bought peacock mantis shrimps from the pet trade in the UK. They presented the purple spot mantis shrimps with a linearly polarised LED light and filmed their eye movements on video camcorders from above. The creatures rotated their eyes and angled specific photoreceptors toward the polarised light source.
Instead of a simple polarised LED light, the researchers showed an expanding circular display on an LCD screen to the peacock mantis shrimps. The crustaceans rolled their eyes again. The researchers say this shows they detect a pattern of polarisation.
The experimenters tilted the screen at different degrees. The peacock mantis shrimps didn’t align their eyes with the angle of polarisation of the foreground or background. Instead, they turned up the contrast between them so objects stood out from the background. “Intuitively, a stable eye should see the world better than a mobile one, but mantis shrimp seem to have found a different way to see more clearly,” says Roberts.
Other creatures, like nocturnal African dung beetles, use polarised light to navigate. They dance on the top of their balls of dung, pivoting their bodies since they can see the polarised light of the night sky only along the rim of one side of their eyes.
The movements of mantis shrimps’ eyes are of a different calibre altogether. The authors say this is “the first demonstration of dynamic polarisation vision mediated by eye movements.” Other creatures cannot change the angle of their receptors like mantis shrimps.
The authors express surprise with another finding. “[I]t is the hemispheres that play a key role in driving rotational eye movements, rather than the mid-band.”
“The mid-band is uniquely able to process colour because of its 12 visual pigments,” Land told The Wire. “It also processes circularly polarised light in a way that makes it much less suitable for processing linearly polarised light. However, the hemispheres have receptors with their microvilli [finger-like structures] at right angles, which is what you need. It comes as no surprise to me that they, rather than the mid-band rows, are responsible for analysing linearly polarised light.” Land wasn’t involved in this study.
Why do these little crustaceans need such complex visual abilities? In their undersea habitat where predators abound, polarised light allows mantis shrimps to maintain a secret communication channel invisible to others. Since their shells reflect polarised light, they can aggressively display to their rivals to back off as well as charm their mates.
“I was pleased to see this paper because stomatopods [mantis shrimps] are known to have polarisation vision and a particularly bizarre range of eye movements,” says Land. “This study brings these two curious features together, with the eye movements being used to maximise the polarisation signal.”
This study solves one piece of the puzzle: “What exactly mantis shrimps are using their ridiculously elaborate eyes for,” says Michael Bok of Lund University, Sweden, who wasn’t associated with this study. “However, there is still much to do. We now know a lot about how their eyes work on the retinal level. The next frontier is to work out their visual sensory neuroanatomy. We need to sort out how they process visual input from the eyes in the brain and then effect any number of their equally sophisticated behavioural modalities. The mantis shrimp certainly seem to be doing visual computation in some very unique ways.”
Based on all that we know about mantis shrimps’ extraordinary eyesight, what do they see? Here’s a glimpse. The authors write that their findings could inspire the development of technology in polarisation cameras and image processing. The study was published in the journal Nature Communications on July 12, 2016.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.