Brand New Eyes

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The Mantis Shrimp (Haptosquilla trispinosa) is not only a beautiful sight, it’s got a punch that can shatter aquarium glass and eyes that function like no other eye in the world (Image source: http://www.the-scientist.com)

Mantis shrimp are undeniably cool. Sporting the fastest punch in the animal kingdom that generates a force 10,000 times stronger than the force of gravity, boils the water in its path, and hits pretty with the force of a rifle bullet, the mantis shrimp is more or less the Liam Neeson of the marine world. A new study out the University of Brisbane and published this week in Science has found that beyond being terrifyingly tough, mantis shrimp also have an entirely unique way of seeing the world.

Mantis shrimp have eyes that sit atop stalks and move independently, and those eyes operate with what is called trinocular vision. Each eye is split into three sections and the comparison of the input from these sections is what allows them to perceive depth. In contrast, humans have binocular vision and our depth perception is based on binocular disparity, where our brains compare the differences from each eye to recognize how far away things are. Since each of their eyes is split into three sections, a mantis shrimp can determine depth better with one eye than a human.

It’s not just structure that makes the mantis shrimp eye unique though. Their remarkable peepers also have twelve photoreceptors to identify colors. In comparison, humans have three, one each for red, green, and blue. Birds and reptiles have four, allowing their vision to extend into the ultraviolet spectrum.  So what are they doing with these extra eight receptors? All kinds of cool stuff.

First off, they can sense polarized light. Light typically moves like a wave, in any and all directions. Polarized light only moves in one direction, like up and down or side to side, or even in a helical pattern like a corkscrew. These patterns of movement are invisible to not only human eyes, but most eyes. But not those of a mantis shrimp. They have special photoreceptors that are able to perceive these wavelengths of light.

Underwater conditions conjure up plenty of polarized light, so being sensitive to that would most likely help a mantis shrimp see better underwater, but there is another possible benefit. As Tsyr-Huei Chiou from the University of Maryland told National Geographic, the mantis shrimp also sports body parts that reflect specific types of polarized light and are commonly used in courtship. So basically, mantis shrimp out cruising for a mate are able to communicate with light signals that cannot be perceived by any other organism. Which is pretty nifty.

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The three sections of the mantis shrimp’s eye allow each eye to perceive depth, while its array of photoreceptors help it to see ultraviolet and polarized light (Image source: phenomena.nationalgeographic.com)

The study published this week in Science was trying to figure out if a mantis shrimp eye was better at differentiating between colors than a human eye (no doubt because of some recent pop culture attention). As it turns out, the overpowered shrimp eye is actually considerably worse at differentiating between colors. Given their uniquely power-packed photoreceptor arsenal, the mantis shrimp should be able to discern between shades that are 1 nm to 5 nm apart on the spectrum of light according to Nature. Instead, the study found that if colors were less than 25 nm apart, the shrimp lost the ability to tell a difference.

The study’s authors are quick to admit that they really don’t know why this is. But they have some ideas. They told National Geographic that their working hypothesis is very simple: mantis shrimp just see color differently than humans. Humans use what is called the “color opponent process.” We use input from our red and green receptors to determine the difference between reds and greens, then the combined input from red and green to determine the difference between blues and yellows. Every other color-sighted animal does the same. The group thinks that rather than comparing inputs from all twelve receptors, which would give an unprecedented clarity of perception, the mantis shrimp just looks at all of them at once. Whichever photoreceptor is reacting most is the one that will define the shrimp’s color perception for each object they encounter.

According to National Geographic, the reasons behind this difference are all still conjecture. It may be that by not comparing input, the mantis shrimp is able to skip intermediary neurons that would slow down color perception, helping to maximize predatory potential. It could also just be that they have small brains. By not requiring those higher cognitive processes of comparison, the mantis shrimp can deal with high amounts of input without developing a larger brain. Regardless of the reason, the mantis shrimp’s eye is unlike any other eye in the known animal kingdom. And that’s just damn cool.

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