Some crustaceans can detect polarized light, using it as a covert signal that is invisible to predators.
Yakir Luc Gagnon at the University of Queensland in Brisbane, Australia, found that the bodies of mantis shrimps (Gonodactylaceus falcatus; pictured) reflect a distinctive pattern of circular polarization (pictured in red) that is visible only to other shrimps. When presented with different burrows in the laboratory, mantis shrimps avoided or delayed entering those that were lit with circularly polarized light compared with those under unpolarized light. This suggests that the shrimps use polarized light cues to sense whether potential burrows are occupied.
In another study, Martin How at the University of Bristol, UK, found that male fiddler crabs (Uca stenodactylus) detected polarized targets in the wild from farther away than non-polarized ones. The animals' sensitivity to polarized light could be boosting the visual contrast between crabs and their mudflat habitat.