Researchers from the Queensland Brain Institute at The University of Queensland have uncovered a new form of secret light communication used by marine animals.
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.”
Funding: This work was supported by the Air Force Office of Scientific Research, Asian Office of Aerospace Research and Development, and Australian Research Council.
Source: Bernadette Condren – University of Queensland Image Source: The image is in the public domain Original Research:Abstract for “Circularly Polarized Light as a Communication Signal in Mantis Shrimps” by Yakir Luc Gagnon, Rachel Marie Templin, Martin John How, and N. Justin Marshall in Current Biology. Published online November 19 2015 doi:10.1016/j.cub.2015.10.047
Circularly Polarized Light as a Communication Signal in Mantis Shrimps
Highlights •Gonodactylaceus falcatus has circularly polarized patterns on its body •It can discriminate unpolarized light from circularly polarized light •This species shows a natural aversion to circularly polarized burrows •G. falcatus may, therefore, be using circular polarization as a covert signal
Summary Animals that communicate using conspicuous body patterns face a trade-off between desired detection by intended receivers and undesired detection from eavesdropping predators, prey, rivals, or parasites [ 1–10 ]. In some cases, this trade-off favors the evolution of signals that are both hidden from predators and visible to conspecifics. Animals may produce covert signals using a property of light that is invisible to those that they wish to evade, allowing them to hide in plain sight (e.g., dragonfish can see their own, otherwise rare, red bioluminescence [ 11–13 ]). The use of the polarization of light is a good example of a potentially covert communication channel, as very few vertebrates are known to use polarization for object-based vision [ 14, 15 ]. However, even these patterns are vulnerable to eavesdroppers, as sensitivity to the linearly polarized component of light is widespread among invertebrates due to their intrinsically polarization sensitive photoreceptors [ 14, 16 ]. Stomatopod crustaceans appear to have gone one step further in this arms race and have evolved a sensitivity to the circular polarization of light, along with body patterns producing it [ 17 ]. However, to date we have no direct evidence that any of these marine crustaceans use this modality to communicate with conspecifics. We therefore investigated circular polarization vision of the mantis shrimp Gonodactylaceus falcatus [ 18 ] and demonstrate that (1) the species produces strongly circularly polarized body patterns, (2) they discriminate the circular polarization of light, and (3) that they use circular polarization information to avoid occupied burrows when seeking a refuge.
“Circularly Polarized Light as a Communication Signal in Mantis Shrimps” by Yakir Luc Gagnon, Rachel Marie Templin, Martin John How, and N. Justin Marshall in Current Biology. Published online November 19 2015 doi:10.1016/j.cub.2015.10.047