Everyone loves the mantis shrimp: It’s one of the most aggressive, quirky creatures evolution has ever engineered. There are 400 separate species, each of them possessing one of two types of claws, “smashers” or “spearers” – both of which are capable of annihilating prey with up to 1,500 newtons (337 pounds) of force within a split second. If this doesn’t sound violent enough, consider this: A new study, due to be published in the journal Current Biology next month, reveals that the mantis shrimp is able to communicate its malevolent intentions using patterns of light.

Previous research by the same team showed that the mantis shrimp Gonodactylaceus falcatus could reflect and detect circular polarizing light, a rare ability in the natural world, although they weren’t sure what it was being used for. Their new study reveals that it is used as a form of warning signal, designed to ward off other hostile mantis shrimp.

Polarization is a property of some electromagnetic waves, including visible light, whereby the electrical field travels up and down as it moves along a line – this is defined as linear polarization. In circularly polarized light, the two electrical fields move forwards at right angles together. If an observer was able to detect this type of light, like the mantis shrimp, it would be seen to be “moving” in a circle, or spiral. 

Careful observation of these marine critters revealed that they have circularly polarized patterns on their body – particularly on the legs, head and their heavily armored tail. When they curl up in a defensive position during conflict, these regions are the most visible. Not only that, but when they are at rest in a burrow, they often display these body parts, making them visible to any other passing mantis shrimp.

^{Image credit: A coiled-up mantis shrimp. Yakir Gagnon/QBI}

“These shrimp live in holes in the reef,” said Professor Justin Marshall, one of the authors of the study, in a statement. “They like to hide away; they're secretive and don't like to be in the open.” 

A tank was designed for a few mantis shrimp to explore; it contained two burrows, one on either side. One was painted in a pattern designed to reflect unpolarized light, and the other contained circular polarizing light patterns. Sixty-eight percent of the time, the mantis shrimp chose the former, probably because the circular polarizing light patterns resembled those found on another mantis shrimp. The mantis shrimp likely thought this burrow was occupied, and moved on to another.

A companion study also by Marshall – due for publication in the same edition of Current Biology – reveals that linearly polarized light is used by fiddler crabs (Uca stenodactylus) as a form of communication. These crabs can detect and identify specific objects depending on how much polarized light is reflected off them; some patterns cause them to be more aggressive, whereas others cause them to be far more defensive.

“In birds, color is what we're familiar with; in the ocean, reef fish display with color. This is a form of communication we understand. What we're now discovering is there's a completely new language of communication,” continued Marshall. “These animals are dealing in a currency of polarization that is completely invisible to humans.”