New form of secret light language keeps other animals in the dark

Researchers have uncovered an entirely new form of secret light communication used by mantis shrimp -

A new form of secret light communication used by marine animals has been discovered by researchers from the Queensland Brain Institute at The University of Queensland. The findings may have applications in satellite remote sensing, biomedical imaging, cancer detection, and computer data storage.

Dr Yakir Gagnon, Professor Justin Marshall and colleagues previously showed that mantis shrimp (Gonodactylaceus falcatus) can reflect and detect circular polarizing light, an ability extremely rare in nature. Until now, no-one has known what they use it for. The new study shows the shrimp use circular polarization as a means to covertly advertise their presence to aggressive competitors.

“In birds, colour is what we’re familiar with and in the ocean, reef fish display with colour–this is a form of communication we understand. What we’re now discovering is there’s a completely new language of communication,” said Professor Marshall. Linear polarized light is seen only in one plane, whereas circular polarized light travels in a spiral–clockwise or anti-clockwise–direction. Humans cannot perceive polarized light without the help of special lenses, often found in sunglasses.

Circular polarized light a form of secret communication

"We've determined that a mantis shrimp displays circular polarized patterns on its body, particularly on its legs, head and heavily armoured tail," he said. "These are the regions most visible when it curls up during conflict." “These shrimps live in holes in the reef,” said Professor Marshall. “They like to hide away; they’re secretive and don’t like to be in the open.”

They are also “very violent”, Professor Marshall adds. “They’re nasty animals. They’re called mantis shrimps because they have a pair of legs at the front used to catch their prey, but 40 times faster than the preying mantis. They can pack a punch like a .22 calibre bullet and can break aquarium glass. Other mantis shrimp know this and are very cautious on the reef.”

Researchers dropped a mantis shrimp into a tank with two burrows to hide in: one reflecting unpolarized light and the other, circular polarized light. The shrimps chose the unpolarized burrow 68 per cent of the time–suggesting the circular polarized burrow was perceived as being occupied by another mantis shrimp. “If you essentially label holes with circular polarizing light, by shining circular polarizing light out of them, shrimps won’t go near it,” said Professor Marshall. “They know–or they think they know–there’s another shrimp there.

Secret light communication may help cancer detection

The findings may help doctors to better detect cancer. “Cancerous cells do not reflect polarized light, in particular circular polarizing light, in the same way as healthy cells,” said Professor Marshall. Cameras equipped with circular polarizing sensors may detect cancer cells long before the human eye can see them. Another study involving Professor Marshall, published in the same edition of Current Biology, showed that linear polarized light is used as a form of communication by fiddler crabs.

Crabs use polarized light to communicate too

Fiddler crabs (Uca stenodactylus) live on mudflats, a very reflective environment, and they behave differently depending on the amount of polarization reflected by objects, the researchers found. “It appears that fiddler crabs have evolved inbuilt sunglasses, in the same way as we use polarizing sunglasses to reduce glare,” Professor Marshall said.

The crabs were able to detect and identify ground-base objects base on how much polarized light was reflected. They either moved forward in a mating stance, or retreated back into their holes, at varying speeds. “These animals are dealing in a currency of polarization that is completely invisible to humans,” Professor Marshall said. “It’s all part of this new story on the language of polarization.”

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University of Queensland   press release

Publications

Gagnon YL et al. Circularly Polarized Light as a Communication Signal in Mantis Shrimps.   Current Biology, Published Online November 12 2015. doi: 10.1016/j.cub.2015.10.047

How MJ et al. Target Detection Is Enhanced by Polarization Vision in a Fiddler Crab.   Current Biology, Published Online November 12 2015. doi: 10.1016/j.cub.2015.09.073