To the human eye, adapted for land, the underwater landscape can appear too dim, too blurry, and too blue. It’s easy to get lost.
To mantis shrimp, however, the ocean environment is richly textured and varied. For a small glimpse of the mantis shrimp’s view of the ocean, humans can now look through a mantis-shrimp-inspired camera from a team led by Viktor Gruev, an engineer at the University of Illinois Urbana-Champaign.
Mantis shrimp have unusual eyes. Mostly famously, they have 16 color receptors, compared to a human’s three. Oddly, they are not that good at distinguishing between colors, but they can detect another property of light invisible to humans: polarization.
While polarization can be difficult to intuit, you can imagine light waves as a bunch of different strings that each have one end attached to a wall. If you shake them randomly, they will vibrate in every direction; that is how non-polarized light behaves. If you only the shake the strings up and down, restricting vibrations to one direction, then that is like vertically polarized light. Light can be polarized in different directions. Polarized sunglasses take advantage of this phenomenon: They reduce glare by filtering out horizontally polarized light that bounces off a road or water surface.
In the ocean, light can be also be polarized when it bounces off molecules in water. Mantis shrimp can see up to six types of polarization: horizontal, vertical, two diagonals, and two types of circular polarization, in which a light wave spirals clockwise or counterclockwise. (They are the only animals known to see circularly polarized light.) Gruev likens it to wearing “six different miniature polarized sunglasses.” So Gruev and his team essentially miniaturized polarized lenses and put them inside a video camera.
Then, they took it underwater. They collaborated with marine biologists who wanted to study how different marine animals use polarization. It’s not just mantis shrimp; the ability to see detect polarization is widespread among cuttlefish, octopus, squid, crabs, and even some fish. Perhaps marine animals use polarization to communicate with each other, or perhaps it enhances contrast underwater for them to detect predators. (You can see what it looks like in the video above above. The polarization has been turned into false color for the benefit of human eyes.)
Marine biologists had been trying to study this by putting a polarizing filter in front of a camera and turning it by hand, taking one photo at a time. Gruev’s video camera could record everything instantaneously and a lot more easily.
Gruev began to notice that the polarized light in the background often changed. This confounded marine biologists, who had believed that light filtering into the ocean could only be horizontally polarized. “The marine biologists were telling us something is wrong with the instrument,” says Gruev, who did not believe the camera was at fault. So began several years of traveling around the world to get more data—in different locations and at different times of the year.
When Gruev traveled for conferences, he took to towing the camera in a heavy-duty suitcase and tacking on an extra day for diving with it. “I would go with the conference and ask for two luggages, and everyone would say, ‘You’re going for three days. You’re bringing extra luggage?’” he says. At one point, he booked a trip to the southern tip of Argentina to study light polarization at polar regions. Unfortunately, he caught pneumonia and only made it to a lake in the north. In the end, he and his team also got polarization data from Hawaii, Australia, Finland, Mexico, and Macedonia.
By looking at the polarization pattern in water and the exact time and date a reading was taken, Gruev realized they could estimate their location in the world. Could marine animals be using these polarization patterns to navigate through the ocean?
The average error when Gruev plotted these locations was 30 miles, which he admitted is large. “Whether or not [marine animals] can use it as a GPS system”—to pinpoint their location on the globe—“I don’t know,” says Roy Caldwell, a mantis-shrimp researcher at UC Berkeley, who was not involved in the study. But the creatures don’t necessary need to know their exact location to navigate using polarization patterns. “I don’t think there’s any question they’re capable of using polarized light as a compass,” says Caldwell. For instance, a shrimp venturing out from its burrow could track the distance it has traveled and turn around until the light-polarization pattern has changed by 180 degrees to find its way back home. This could be especially helpful in the featureless, muddy plains where some species live.
There are, in fact, animals on land that navigate using polarization. (Light bouncing off molecules in the atmosphere can become polarized, too.) Honeybees do this. As do dung beetles rolling their balls of dung to polarized light from the moon and even Milky Way. James Foster, who studies dung beetles at Lund University, says the study is “a nice proof of concept that is a long time coming.” In fact, he notes, it might have helped him crack his old Ph.D. project, which investigated damselfish and polarization. He had been studying how the damselfish use it to detect predators, but maybe it was navigation that was important all along.
Foster hopes the new camera will inspire a lot more research in underwater polarization. It helps that marine biologists can finally start to see the ocean the way the animals they study see it.