Shrimp eyes may hold key to better communications

Scientists have found a fourth mode of sight — joining black and white, color and linearly polarized — in the mantis shrimp, a crustacean that's taken a unique evolutionary path for 400 million years.
Photo: Roy Campbell

Scientists have discovered a new mode of sight in a most unusual creature: a primitive marine crustacean known as the mantis shrimp.

Until now, it was believed that animals could see only the intensity, color and -- in a few species -- the linear polarization of light.

But in a study published today in Current Biology, researchers showed that the mantis shrimp, a fancifully nicknamed stomatopod whose unique evolutionary path began some 400 million years ago, perceives what's known as circular polarized light. They are the first organisms to demonstrate this ability.

Linear polarization refers to light with photons traveling along parallel, up-and-down wavelengths. By contrast, circular polarized light -- CPL for short -- has photons traveling in parallel, rotating wavelengths. It occurs rarely in nature but is widely used in high-tech communications. Researchers say that understanding the eyes of mantis shrimps could help engineers design better CPL systems.

Many satellite systems rely on circular polarized light, as do devices that amplify radio signals, because CPL's tightly rotating configuration reduces data losses during transmission. (When an antenna sports something that looks like a tightly wound spring, that's for CPL.) It could someday be used in mobile phones -- invisible to our eyes, but not to the machines that detect it.

Human applications aside, the discovery of a new type of sight is a landmark unto itself, and the mantis shrimp is an odd but deserving history maker.

"Since they branched off from animals so early, their evolution took them in unique directions," said study co-author Tom Cronin, a visual physiology researcher at the University of Maryland, Baltimore County. "We used to call them shrimps from Mars, because their biology is so different from any other animal."

Like insects and other crustaceans, mantis shrimps possess compound eyes composed of thousands of rows of light-detecting units called ommatidia. These are especially refined in mantis shrimps, containing a mix of photoreceptors and filters that let them see 100,000 different colors -- 10 times more than can be detected by humans.

Two decades ago, Cronin, along with co-authors Justin Marshall at the University of Queensland and the University of California, Berkeley's Roy Caldwell, noticed that sections of the mantis shrimps' ommatidia are arranged at a slant.

This suggested an ability to detect circular polarized light, in which photons follow a corkscrew path and ostensibly enter the ommatidia at a correspondingly slanted angle. After finding a species that seemed to send signals with a CPL-reflecting patch of exoskeleton, the researchers decided to test whether the shrimps' oddball ommatidia really registered the light.

First they hooked severed eyes to electrodes to measure whether the cells energized when hit with circularly polarized light; they did. Then they trained the shrimps to associate CPL-reflecting boxes with food. The shrimps passed the test with flying colors.

Cronin said the shrimps probably use CPL to communicate during sexual and territorial encounters, though he doesn't know why they evolved such a one-of-a-kind system. Further research may illuminate those origins -- and, Cronin said, could help scientists refine their use of CPL in computer screens and signal transmission, where its tightly rotating configuration lends itself to loss-free transmission.

"We're now looking at the molecular diversity of the visual pigments that create these visual analysis cells," said Cronin, whose lab is partly funded by the signal-hungry Air Force.

Not everyone was so enthused about the practical applications of the shrimps' vision.

"The mantis shrimp system is pretty much the way it is done in optical technology, so I'm afraid there is not a lot to be learned here," said Mike Land, a vision researcher at the University of Sussex.

But Justin Marshall disagreed.

"We invented this a few years ago, and stomatopods [invented it] 400 million years ago," he said, noting that CPL is used in detecting skin cancer. "They have honed and perfected a system that we could learn from."