New type of vision found in shrimp

Source: University of Queensland

University of Queensland research has identified a completely new type of vision never seen before in the animal world - and it is probably used for sexual purposes.

Professor Justin Marshall, from UQ's School of Biomedical Sciences and the Queensland Brain Institute, has found the common mantis shrimp, also known as stomatopods, sees light in a way that is vastly different to other animals.

"We are sensitive to light intensity and colour, but we need cameras and filters to allow us to see different types of light," Marshall said.

"To find out these animals can see circular polarised light, it is as if we had discovered colour vision for the first time - it is quite a breakthrough."

He describes circular polarised light as a spiralling beam that spins either to the left or the right. Some animals such as scarab beetles reflect this kind of light but mantis shrimp are the only known ones who can see it.

The next question for Marshall's team after discovering this unique talent was to find out what it was used for.

"It really is bizarre why these animals even bother with this type of vision," he said.

"But after doing a number of tests over the years looking at the physiology and behaviour of mantis shrimps, we have come to the conclusion it is used for sex.

"Only the males have this ability, implying it has something to do with sex.

"It is probably some secret communication channel between males and females while at the same time preventing predators from knowing what was going on."

"The precise role that these signals, visible to a CPL (circular polarised light) visual system, play in stomatopod sexual signalling is not yet known, but we speculate that these CPL reflections could act as a secret communication channel," the researchers write.

"Linear polarisation signals, used by marine invertebrates, are visible to animals like cephalopods that prey on stomatopods and are therefore open to exploitation.

"Also, other genera of stomatopods that we have examined have variable CPL sensitivity, and may be unable to view the sexual displays of Odontodactylus species, making this a private channel of communication, unavailable to both predators and potential stomatopod competitors."

Marshall said the mantis shrimp was an amazing animal to study as it has a very small brain but one of the world's most complex visual systems.

"Us humans only have three colour channels," he said.

"These little guys have 12, and can see both linear and circular polarised light - it is remarkable."

He said humans could see the effect of linear polarisation through things such as polarised sunglasses, which cut out glare for driving and boating.

Circular polarisation is also used by humans in things such as 3D glasses for movies, filters for cameras and medical imaging systems to detect skin cancer.

"It's quite amazing to think how much circular polarisation technology we have, and that 400 million years ago nature got there first with a mantis shrimp's eyes," he said.

The research is published in the March 25 edition of Current Biology.