Marine animals use new form of secret light communication

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 polarising 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 polarisation as a means to covertly advertise their presence to aggressive competitors.

"In birds, colour is what we're familiar with; 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 polarised light is seen only in one plane, whereas circular polarised light travels in a spiral -- clockwise or anti-clockwise -- direction.

The team determined that mantis shrimp display circular polarised patterns on the body, particularly on the legs, head and heavily armoured tail; these are the regions most visible when when they curl up during conflict.

"These shrimp 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."

Researchers dropped a mantis shrimp into a tank with two burrows to hide in: one reflecting unpolarised light and the other, circular polarised light. The shrimp chose the unpolarised burrow 68% of the time -- suggesting the circular polarised burrow was perceived as being occupied by another mantis shrimp.

"If you essentially label holes with circular polarising light, by shining circular polarising 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.

The findings may help doctors to better detect cancer. "Cancerous cells do not reflect polarised light, in particular circular polarising light, in the same way as healthy cells," said Professor Marshall. So cameras equipped with circular polarising sensors may detect cancel 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 polarised light is used as a form of communication by fiddler crabs.

Fiddler crabs (Uca stenodactylus) live on mudflats, a very reflective environment, and they behave differently depending on the amount of polarisation reflected by objects, the researchers found.

"It appears that fiddler crabs have evolved inbuilt sunglasses, in the same way as we use polarising sunglasses to reduce glare," Professor Marshall said.

The crabs were able to detect and identify ground-base objects base on how much polarised 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 polarisation that is completely invisible to humans," Professor Marshall said. "It's all part of this new story on the language of polarisation."

Both studies will be published in the print edition of Current Biology in December 2015.