Mantis shrimp’s bizarre visual system may save brainpower

No one can deny that the mantis shrimp is special. The charismatic crustacean looks like a walking Mardi Gras parade and hammers its enemies so fast that water boils. Now scientists have added another distinction: The mantis shrimp has a really strange way of seeing colors.  

People and other animals perceive a palette of colors by blending and comparing signals from a few types of color-sensing eye cells called photoreceptors. In contrast, the mantis shrimp sees each color separately with one of a dozen kinds of specialized cells, scientists suggest in the Jan. 24 Science.

The bizarre color vision system might be a way for the mantis shrimp to quickly see colors without a lot of brainpower. Because mantis shrimp don’t have big brains, their dazzlingly complex eye might do the bulk of their color processing, says vision scientist Michael Bok of Lund University in Sweden, who wasn’t involved in the study.

But the rapid color processing in the eye comes with a price, he says: The animal is actually pretty bad at seeing color (SN: 9/22/12, p. 11).

Instead of working together, each of the 12 types of photoreceptor in the mantis shrimp seems to work alone, rendering the animal surprisingly bad at telling apart colors, says study coauthor Justin Marshall, a vision neuroscientist at the University of Queensland in Brisbane, Australia. “You’d expect an animal that has four times more color photoreceptors than us would be good at seeing color,” Marshall says.  Given the mantis shrimps’ poor performance, he says, “it’s clear that they’re doing color vision very differently.”

Marshall and his colleagues studied mantis shrimp (Haptosquilla trispinosa) that had learned to swipe at a particular hue of optical fiber that had been paired with food. After that food-linked color became a favorite, the team added a second optical fiber of a different color and watched as mantis shrimp scuttled out of their burrows and grabbed at a color. The team continued to offer pairs of fibers including the favorite and a new color. As the two hues grew more similar, mantis shrimps’ performance worsened.

When the wavelengths of the two colors grew closer than 25 nanometers (to people, that represents the difference between pure yellow and orange), the animals started to have trouble identifying the food-linked fiber. For some color choices when the difference shrank to 12 nanometers, the mantis shrimp did no better than chance.

Marshall and colleagues think that mantis shrimp detect colors by shunting each wavelength of light into one of 12 narrowly defined bins, creating an almost pixelated representation of color. If this color discrimination happens largely in the eye without much brain processing, the system might save time, allowing for fast decisions in the violent life of a mantis shrimp, Marshall says.

Scientists don’t yet know how the signals from the 12 photoreceptors translate into action. There must be some unknown complexity, says vision scientist Daniel Osorio of the University of Sussex in England. Otherwise, the animals would fail to recognize a color as the same in different lighting conditions.

And it’s possible that the mantis shrimp are better at telling apart colors in their everyday lives than their dismal lab performance would suggest. “One concern when you have poor performance is that you’re not testing in a way that allows the animal to do its best,” Osorio says.

The team plans on tracing the neural wiring that carries color signals from the eye to the brain, in the hopes that anatomy might help them untangle the mantis shrimp’s strange visual system. “They’re confusing animals,” Marshall says.