27 October 2009—Scientists have discovered the mechanism behind the eyes of the only animal that can detect a certain kind of polarized light: the mantis shrimp, native to Australia's Great Barrier Reef.
Optical devices can manipulate the polarization of light for research and for commercial products like CD and DVD players and digital cameras. However, these devices can't manipulate light nearly as well as the mantis shrimp can, says biologist Nicholas Roberts of the University of Bristol, in England. Roberts, together with scientists at the University of Maryland, Baltimore County, and the University of Queensland, Australia, reported the finding yesterday in the journal Nature Photonics.
It's been known that some animals, including humans, have a slight ability to perceive the linear polarization of light. Linear polarized light has an electric field oriented in only one direction or plane. But the mantis shrimp's sophisticated eyes do much better. They can distinguish between left- and right-rotated circularly polarized light—where the orientation of the electric field rotates as the light travels—a unique ability in nature. The creature's eyes can also convert linearly polarized light into circular polarization, and vice versa.
That principle of converting between linearly and circularly polarized light is used in some of the most common optical devices today, Roberts says. But man-made devices are limited to manipulating polarization at a single wavelength, while the mantis shrimp—properly called a stomatopod—can achieve this feat across the visible spectrum, from blue to red.
"What's amazing about this is the wavelength range," says MIT electrical engineering and computer science professor Shaoul Ezekiel. "With the polarization materials we use, some work beautifully at one wavelength." But when you try to expand the range to other wavelengths, he says, the materials don't behave the same way. "We can play games with mixing materials" to rotate light the way we want, says Ezekiel, "but this son of a gun seems to be able to do it for a wider range of wavelengths than humans can do."
Last year, researchers reported the first evidence of the mantis shrimp's ability to distinguish between two circular polarizations of light, something not seen in any other animal. Now they've found out how it works. The stomatopod's eyes are split into an upper and a lower hemisphere, with a series of ribs in between. It's the bottom two ribs in that central band of the eye where polarization detection happens, the researchers discovered.
Inside these ribs, the cells that detect polarization can be imagined like a bundle of tubes, similar to the bristles on a toothbrush. The researchers measured the structural properties of the cells to determine the dimensions of the tubes, the path lengths through which light travels, and the indices of refraction of the cell membranes. Then they did some optical modeling with the structure to determine how it turns linear polarization to circular polarization, or vice versa.
The interesting thing about this ability is that the mantis shrimp's eyes have evolved to work both as traditional photoreceptors and "as these polarization devices," Roberts explains. Over time, he says, the dimensions of the tubes and the amount of lipid material inside them may have changed, resulting in slightly altered optics that "can add a whole different capability" to the vision system. A man-made equivalent requires multiple devices, such as separate detectors and polarizers, to accomplish the same feat.
The researchers suggest that manufacturers could possibly mimic this exotic design by re-creating the optical properties and dimensions of the mantis shrimp's eye tubes. Then they could potentially make polymer films that help CD or DVD players read information at multiple wavelengths, or make better circular polarizing filters for digital cameras, which would reduce glare from water or the sky, improve color contrast, and create sharper images.
If the mantis shrimp eyes prove too difficult to engineer, they could even be harvested from the real thing, Ezekiel says, jokingly. "You could take a bunch of these eyes and line them up."