COLLEGE PARK, Md. — A mantis shrimp, which has one of the most elaborate visual systems ever discovered, turns out to be pretty lousy at distinguishing one color from another.
The puzzling underachievement may mean that the mantis shrimp brain perceives color in a way new to science, says Hanne Thoen of the University of Queensland in Brisbane, Australia. She presented results from her ongoing study August 6 at the 10th International Congress of Neuroethology.
The stalked eyes of mantis shrimp species that live in shallow water can have up to 16 kinds of photoreceptor cells, 12 of which are specialized for different colors. People make do with four kinds, three of which pick up colors.
Yet tests with pairs of increasingly similar colors found that the mantis shrimp Haptosquilla trispinosa flunks out when choices narrow to colors 15 nanometers apart in wavelength, Thoen said. At sweet spots in the color spectrum, people can distinguish between colors only 1 or 2 nanometers apart.
“Hanne’s results are a bit of a shock to us,” says Thomas Cronin of the University of Maryland, Baltimore County, whose lab also studies mantis shrimp vision.
Thoen tested the color vision of mantis shrimp by training them to scoot out of their burrows toward a pair of optical fibers and punch at the one glowing a particular color. As she narrowed the color gap between the two fibers, she could tell when the animals no longer discerned a difference.
So far, Thoen has tested her mantis shrimp on six target colors ranging from a 425-nanometer purple to a 628-nanometer red. If the animals perform just as poorly at distinguishing colors in other wavelengths, then mantis shrimp may be using some unknown system of color perception.
People and other animals studied so far distinguish colors through brainpower by interpreting competing activity in different kinds of light-receptor cells. Instead of doing such fancy brainwork, mantis shrimp may just rely on what a particular specialized cell responds to strongly. Wavelengths that tickle the purple-sensitive cells may be just plain purple regardless of whether they’re more toward the blue or the ultraviolet.
Mantis shrimp species do see into the ultraviolet, which people don’t. It’s possible that such a simplified light system would work in this range too because the light-catching receptors don’t overlap much in sensitivity, speculates Michael Bok of the University of Maryland, Baltimore County.At the meeting, he described the workings of six different kinds of cells that respond to UV light in the mantis shrimp Neogonodactylus oerstedii . Even though the receptors each detect a relatively small slice of the UV spectrum, Bok found that just two light-sensitive pigments provide the basis for all those receptors. Filters in the cells create the array of different sensitivities, he said.