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A fierce crustacean known as the peacock mantis shrimp has eyes so refined they can perceive polarized light, including information that is invisible to nearly every other member of the animal kingdom. Not only can the ocean dweller extract polarization information from light, it can do so when the light is circularly polarized—an ability unknown outside a few species of the order of stomatopods to which the peacock mantis belongs.
Unlike linearly polarized light, in which the electric field oscillates along a plane, circularly polarized light's field twists like a spiral spring as the ray propagates. Such light is not commonly reflected from animal bodies and so was long dismissed as a virtual nonfactor in physiology, but research last year showed that some stomatopods have the ability to discriminate circular polarization. A paper published online October 25 in Nature Photonics unpacks the mechanism behind the mantis shrimp's ability and concludes that its eyes handle circularly polarized light more effectively than man-made optical devices do. (Scientific American is part of the Nature Publishing Group.)
The peacock mantis, or Odontodactylus scyllarus, packs a surprisingly powerful punch for its size. The crustacean, which ranges from three to 18 centimeters in length, is capable of shattering the glass of an aquarium with a blow from its forelimb, says Roy Caldwell, a University of California, Berkeley, biologist who did not participate in the new research. "We have had a couple cases where animals have hit a pane of glass dead in the center and there was a massive explosion," Caldwell says.
But the creature is physiologically remarkable in at least one other way: The compound eye of the peacock mantis, the new study's authors found, harbors a natural quarter-wave retarder, a sort of filter that converts circularly polarized light to linearly polarized light, which then activates receptors below. "Biologically, this is unique," says study co-author Thomas Cronin, a professor of biological sciences at the University of Maryland, Baltimore County. "There is nothing else known anywhere in biology" that enables detection of circularly polarized light, he adds.
The stomatopods reflect circularly polarized light from their bodies, so their ability to detect such light—and to parse clockwise from counterclockwise polarization—likely plays a role in signaling or identification. In some stomatopod species, reflection of circularly polarized light is sex-specific, which could play a role in sexual signaling or mate selection.
Wave retarders work by refracting light differently depending on the angle of its polarization, delaying one wave component of a light wave relative to the other. "If it's just the right degree of delay, which is one-quarter wave or 90 degrees phase, that converts circularly polarized light to linearly polarized," Cronin explains. But unlike wave retarders available commercially, which are tuned for specific wavelengths (and hence colors) of visible light, the wave plate in the O. scyllarus eye performs almost identically across the visible spectrum.
The mantis shrimp's eye, Cronin explains, "works on a principle that is not used currently but could be used in manufacturing systems"—balancing the optical properties of the eye structure with those of the lipid molecules that fill the structure. "The two have different wavelength functions—they have different curves of changing retardance with wavelength—and so the animal trades them off," Cronin says. "It trades off structure against material to cancel out the two variations."
Sonja Kleinlogel, a biologist at the Max Planck Institute of Biophysics in Frankfurt am Main, Germany, points out that she and a colleague published a similar analysis last year in the journal PLoS ONE—an article that she was surprised to see omitted from the references section of the new paper. Nevertheless, she is pleased to see the subject advanced, noting that the research "is the first to look at the detailed structure" of the cells that act as quarter-wave retarders and to compare their efficacy with man-made analogues. U.C. Berkeley's Caldwell concurs, noting that the unique capability of the stomatopod eye had been described but "how that actually was done was pretty much a mystery."
"We didn't know anything about the operating principle of the retarder," Cronin says. "It wasn't like anything we had seen in the lab."
