In the oceans, secret messages are being exchanged all the time. Some are obvious if you look closely enough, or at the right angle, or with sharp enough eyes. But mantis shrimps—a group of belligerent crustaceans—exchange signals that are so secretive that only they can see them.
Mantis shrimps have arguably the weirdest eyes in the oceans. Each eye can focus on objects with three separate regions, giving the animal ‘trinocular’ depth perception with a single eye. Some species have 12 types of light-sensitive cells, each tuned to a different colour; we have just three. They can even use filters to tune these cells to local light conditions.
Their eyes can also detect polarised light, which vibrates in a single fixed plane—imagine that you’ve attached a string to a wall and are shaking it up and down. We can’t see this kind of light, but many animals can. Aquatic insects like backswimmers use it to find bodies of water, because such bodies reflect polarised light from their surfaces. Other insects use patterns of polarised light in the sky to find their way. Octopuses and cuttlefish use it to fine-tune their camouflage, and possibly even to communicate.
But polarised light doesn’t always travel in a flat plane. It can also move in the shape of a helix, as a spiralling beam. This is circular polarisation, or CP. And in 2008, Justin Marshall at the University of Queensland showed that mantis shrimps are the only animals that can see it. For example, they tell the difference between CP that’s spiraling in a clockwise or anti-clockwise direction.
But why? The team found one important clue: It seemed that the shells of mantis shrimps reflect CP, and that males and females have different patterns. Perhaps the animals use these patterns to send clandestine communiqués to one another during courtship. Still, to date, “no one has ever shown an example of [mantis shrimps] using CP for anything,” says Yakir Gagnon, a postdoc in Marshall’s group.
Video: World’s Deadliest—Shrimp Packs a Punch. A mantis shrimp breaks through quarter-inch-thick glass to grab a meal.
He set out to find the first such example, by working with a small species of mantis shrimp that has clear CP patterns on its tail, legs, and head. When these animals fight each other—and they do that a lot—they curl up to shield their bodies with their heavily armoured tails. In these postures, all the CP-reflecting surfaces are clearly visible. So, maybe these patterns are used as warnings or threats?
Gagnon tested this idea by exploiting the fact that mantis shrimps, when left out in the open, will unerringly seek the shelter of a burrow. It’s better for them if said burrow is empty; if there’s another mantis shrimp inside, the two will have to fight, and given how powerfully these animals can punch, such bouts can be very dangerous.
So, Gagnon dropped mantis shrimps into a circular tank and gave them a choice of two burrows, both of which were blocked by filters shaped like mantis shrimp tails. One filter reflected unpolarised light, and the other reflected circularly polarised light. The animals showed a clear preference for the unpolarised burrow, running towards it 68 percent of the time.
They did the same if Gagnon backlit the burrows with either unpolarised or CP light. This time, the mantis shrimps chose the unpolarised burrow 88 percent of the time. And those that headed towards the CP burrows spent 7 times longer inspecting them before trying their luck.
These results suggest that mantis shrimps—or, at least, this particular species—use their CP patterns as signs that say “Occupied”. To another mantis shrimp, the message would be like a flashing neon sign. To a passing fish or octopus, it would be completely invisible; they’d see a green and brown shell, perfectly camouflaged against the rocks of a coral reef.
“We haven’t seen [CP vision] in any other animal,” says Gagnon. Fireflies and scarab beetles are the only other groups that reflect CP, but there’s no evidence that they can see it, or have the special eye structures that allow mantis shrimps to do so. “It’s just so rare.”
Mike Bok from the University of Lund, who studies mantis shrimp vision, says he was initially skeptical about the results. “As someone who has spent a lot of frustrating time working on mantis shrimp behaviour, an 88 percent preference for unpolarized light seems suspiciously high for these erratic animals,” he says. “However, Yakir has supported his results well, and I now believe that the high preference speaks to the great significance of CP signals to these animals.”
Roy Caldwell from the University of California, Berkeley also praises the study. “Trying to ascribe a function to CP is still difficult, but I think they’ve come closer than we have in the past,” he says. But he adds that since even the most primitive species of mantis shrimps can see CP, “this channel of communication has been around for a very long time and many different potential uses could have evolved.”
Indeed, Marshall’s team is now testing other possible uses, including signalling during courtship (as they first suggested), enhancing visual contrast, and even assessing the properties of materials they want to hit.