Scientific Method —

One of the strangest animals on earth gets a little weirder

Despite having 12 photoreceptors, mantis shrimp aren't great at sensing color.

Roy L. Caldwell

Mantis shrimp, the psychedelic reef-dwellers that can wallop their prey with an astounding 200 pounds of force, have a large collection of unique qualities. One is an unusually large number of photoreceptors, the light-sensing proteins that contribute to color vision. Humans have three types of color receptors, birds and reptiles have four, and mantis shrimp have an astounding 12 different kinds.

Each type of photoreceptor samples a small set of wavelengths in the color spectrum, but our vision demonstrates that just three or four channels are sufficient to distinguish between different hues even on a very fine scale. Why, then, would these creatures need 12 types of photoreceptors?

According to a new study in Science, mantis shrimp may use a vision system previously unknown to science. This key to this realization was the finding that despite having so many different photoreceptors, the animals have trouble distinguishing between similar colors.

If you’re wondering how to determine whether an animal can discriminate between, say, burnt orange and dark yellow, you probably aren’t alone. But it’s surprisingly simple to test animals’ color vision: just teach a bee, dog, or mantis shrimp that a specific color delivers a food reward, then offer that animal an array of colors to choose from. If the animal is capable of distinguishing between certain colors, it will approach the rewarding hue more often than would be expected by random chance.

The authors of the study used this time-tested method to determine the limits of mantis shrimps' color discrimination abilities. The shrimp were able to accurately distinguish between different colors of light when the wavelengths were between 50 and 100 nanometers apart. But when the colors were more similar—the wavelengths differed by just 12 to 25 nanometers—the shrimp’s accuracy dropped to about 50 percent, or what would be expected if they were choosing randomly.

As a comparison, humans can distinguish between colors with wavelengths less than five nanometers apart. So the mantis shrimp's complex visual system clearly doesn’t provide enhanced color discrimination abilities. Instead, its many photoreceptors may facilitate a completely different color vision system based on recognition rather than discrimination. Almost every animal on Earth—including humans—identifies colors by comparing the output of its various photoreceptors sequentially. The study’s results suggest that mantis shrimp don’t seem to be seeing color in the same way; instead, they appear to skip this time-consuming comparison step completely.

While the researchers aren’t sure exactly what the shrimp are doing instead, it’s possible that their visual system functions somewhat like a satellite sensor. The eyes of a mantis shrimp move independently—like chameleons’ eyes—and are perched atop stalks on their heads. To see, the shrimp move their eyes across whatever they are looking at, scanning it sequentially with their visual cells.

The eyes of the mantis shrimp in action.

The pattern created by this scan may allow shrimp to recognize color without a lengthy comparison process, cutting down on the time and the neural processing required by other systems. It’s a process that could improve our own technology: think quicker CDs and DVDs or super-efficient bioinspired satellites.

So far, mantis shrimp are the only animals known to use this type of color vision, and the researchers believe that it may help these colorful crustaceans respond quickly to challenges they face in the vibrant coral reefs where they live. Mantis shrimp are ferocious fighters that rely on visual cues to figure out their next move; having a lightning-fast color recognition system with minimal neural processing might make the difference between giving and receiving that 200-pound wallop.

Science, 2014. DOI: 10.1126/science.1245824  (About DOIs).

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