"Weird Beastie" Shrimp Have Super-Vision

Anne Minard
for National Geographic News
May 19, 2008
The mantis shrimp is known to see colors invisible to humans and other animals, viewing the world in 11 or 12 primary colors, as opposed to our humble 3.

Now a new study has found that the shrimp also have optimal ability to see different forms of light polarization—directions in which light vibrates.

Humans have been able to see polarized light only within the past decade, and only with the aid of technology, such as digital cameras and polarizers.

"The mantis shrimp is a delightfully weird beastie," said study co-author Andrew White of the University of Queensland in Australia.

"And now we find that this species can see a world invisible to the rest of us."

(Related: "Mice Get 'Human' Vision in Gene Experiment" [March 22, 2007].)

Beyond Rainbows

Most animals can tell how fast the electric field in a light wave is oscillating, which is perceived as color.

Blue light oscillates, or vibrates, faster than green, for example, and both of those colors are faster than red. The oscillation's direction is known as polarization.

Many animals, from parakeets to ants, can see linear polarization—what happens when light follows different angles after it's reflected off of water or a transparent membrane such as a fly's wing.

Bees and some birds navigate by polarized skylight, and fish and crustaceans use polarization for navigation and seeing prey.

But mantis shrimp are the first animals known to see both linear polarization and a much rarer type called circular polarization.

Linear polarization occurs when an electric field of light oscillates back and forth in a straight line. In circular polarization the light oscillates around and around in a circle.

The research, led by Sonja Kleinlogel of the Max Planck Institute of Biophysics in Frankfurt, Germany, appeared recently in the online journal PLoS ONE.

But Why?

Some scientists have speculated that circular polarization vision is used for sexual signaling.

But study author White said, "We found the same structures in the eyes of both 'boy' and 'girl' mantis shrimps, and yet neither have circularly polarized markings on their bodies."

He believes it makes more sense that mantis shrimp evolved such advanced abilities to detect prey.

"Some of the animals they like to eat are transparent and quite hard to see in seawater," White said.

"I suspect [their prey] light up like Christmas trees as far as these shrimp are concerned."

Rare Ability

Raymon Glantz, a professor at Rice University in Texas, called the new study "a first-rate paper." But he's not sure how important the discovery will turn out to be, because circularly polarized light is so rare in nature.

So far, only a handful of sources of circularly polarized light have been found: firefly lanterns, scarab beetle bodies, chlorophyll, and crustacean bodies—including those of mantis shrimp.

"It opens up the possibility that we have in crustaceans complete polarization analyzers," he said.

"What the significance of that is biologically—that's not clear."

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