Mantis Shrimp Eyes See Through A One-Stop Process: Crustacean's Photoreceptors Don't Send Visual Info To Brain

By Gabrielle Jonas on January 24, 2014 3:09 PM EST

Always in a Hurry: So Much to See
The mantis shrimp has more types of photo-receptors in its retinas than any other creature, and marine biologists think they finally know why. The shrimp don't want to waste time sending images to their brains, when they can just scan their own eyes, with, well, their eyes. (Photo: Wikipedia)

Marine biologists have long been mystified as to why mantis shrimp, otherwise known as stomatopod crustaceans, boast up to 12 different types of photoreceptors in their retinas — more than any animal in the kingdom. A team of marine biologists from the Queensland Brain Institute, Brisbane, and National Cheng Kung University,Taiwan, became even more perplexed when they tested the shrimps and found that, even with all those extra receptors, the stomatopods couldn't tell the difference between gradations of colors, which humans, with their paltry three types of receptors can easily pick out within just a few nanometers difference of each other. But after thinking about it for a while, the biologists said in Friday's issue of Science, they had finally figured it out.

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Each type of receptor in a mantis shrimp samples a narrow set of wavelengths, ranging from deep ultraviolet to far red. "This chromatic complexity has presented a mystery," the authors wrote. "Why use 12 color channels, when three or four are sufficient for fine color discrimination?" Moreover, when they used colors to lure the shrimp toward food rewards, they made "a surprisingly poor performance," causing the biologists to rule out the conventional color-opponent coding system found in the brain. "Instead," wrote the biologists, "our experiments suggest that stomatopods use a previously unknown color vision system based on temporal signaling combined with scanning eye movements, enabling a type of color recognition rather than discrimination."

The stomatopod seems to identify color by making "a temporal scan of an object across the 12 photoreceptor sensitivities," according Science, effectively using each type of receptor as a different color reference point. "This entirely unique form of vision would allow for extremely rapid color recognition without the need to discriminate between wavelengths within a spectrum."  So the shrimp can "see" colors directly without referencing them in its brain.

It's not clear yet whether the marine crustacaeons (which can grow up to a foot long) have to move so quickly to survive attacks, that they have to bypass brain function for that extra time advantage. Not only would this be an aid to defense, but it may be one of the  reasons that mantis shimps are so lightning-quick with their attacks: They can process "potential meal" so quickly that they their prey has no time to make a quick getaway.

It seems that the mantis shrimp is always in a hurry. Not only does it have to bypass brain vision, because, well, waiting for the brain to analyze data is just too much of a drag, it has to kick more swiftly than any other known predator on the planet. About 10 years ago, University of California, Berkeley scientists recorded the swiftest kick of any predator. The shrimp smashed their club-shaped leg at speeds of up to 23 meters per second to shatter the snail shells and the occassional pane of tnak glass. "The speed of this strike exceeds most animal movements by far," said biologist Sheila Patek, a Miller Post-doctoral Fellow at UC Berkeley at the time. "It's insanely fast, but important for generating the forces necessary to crush its preferred food." It seems that when it comes to seeing or punching, these crustaceans are built for speed. 

They are also inspiring new technologies. Researchers investigating other aspects of the shrimps' vision say their many of their photoreceptors may have less to do with food, and more to do with courtship. Six years ago, marine biologist Justin Marshall of the University of Queensland in Australia suggested the ability to perceive circular polarized light may lend mantis shrimp a secret mode of sexual signaling; "a private channel of communication, unavailable to both predators and potential stomatopod competitors."  It was known that some animals, such as scrab beatles, reflected circular polarized light as a spiraling beam that spins either to the left or the right. But no one knew there was a creature that could actually see that light naturally, until Marshall proved they could. The stomatopods had filters in their eyes like little ledges, conveniently oriented at 45 degree angles to their photoreceptors, thus handily turning the circularly polarized light into its linear form. Without those ledges, humans experience that kind of light as glare: Hence the need for polarized sun glasses. Marshall has identified three species of stomatopods where circular polarized light is reflected from the cuticles of males only, right where the males use them for behavioral displays.  

This ability is so impressive that the eye of the peacock mantis shrimp has led an international team of researchers to develop a two-part waveplate that could improve CD, DVD, blu-ray, and holographic technology by creating even higher definition and larger storage density. Peacock mantis shrimp are one of only a few animal species that can see circularly polarized light - like the light used to create 3-D movies. Some researchers believe the mantis shrimp's eyes are better over the entire visual spectrum than any man-made waveplates, which are transparent slabs that alter the polarization of light.

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