Mantis shrimps use it to avoid already occupied hiding places.
From ScienceDaily:
Researchers from the Queensland Brain Institute at The University of Queensland have uncovered a new form of secret light communication used by marine animals.
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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.
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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.
“It appears that fiddler crabs have evolved inbuilt sunglasses, in the same way as we use polarising sunglasses to reduce glare,” Professor Marshall said. Bl21The 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.” More.
Which all just sort of happened, right? Pieces all just landed in place via Darwin’s Lotto.
See also: Life as “self-perpetuating information strings”? At least Adami is on the right track in focusing on understanding information, not chemistry, as the key driver.
and
What we know about how animals think (or whatever it is that they do)
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Here’s the abstract:
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. (paywall) Yakir Luc Gagnon, Rachel Marie Templin, Martin John How, N. Justin Marshall. Circularly Polarized Light as a Communication Signal in Mantis Shrimps. Current Biology, 2015; DOI: 10.1016/j.cub.2015.10.047