The world drips with color because the human eye has three types of so-called cone cells, which sense red, blue, and green light. But our vision is downright dull compared to that of the mantis shrimp. Besides holding the world record of eight kinds of cells that receive visible light, these shrimp have receptors for no less than four different wavelengths of ultraviolet, allowing them to sense a broader palette of colors than any other animal.
A decade ago, biologist Justin Marshall of the University of Queensland in Brisbane, Australia, was studying the anatomy of the eyes of mantis shrimp, gaudy inhabitants of tropical coral reefs. Like all arthropods, these shrimp have eyes made of geometric arrays of tiny lenses that focus light. Marshall discovered that the cells underneath four rows of lenses across the center of the eye were specialized for color vision. When he shone light into individual lenses, the retina absorbed eight distinct bands of wavelengths--implying the presence of color filters and eight color receptor proteins.
Given that the microscopes he used for his measurements were only good for studying how visible light impinges on the retina, Marshall teamed up with Johannes Oberwinkler of the University of Groningen in the Netherlands to test for ultraviolet sensitivity. Oberwinkler stuck minuscule electrodes into retinal cells and measured the currents generated as incoming light of various wavelengths gets absorbed by individual cells. Sure enough, besides confirming the shrimp's keen eyesight in the visible light, some receptors responded to four colors of ultraviolet, they report in yesterday's Nature.
The mantis shrimp appear to have developed what the researchers have dubbed "digital" color vision. The 12 kinds of receptors and their filters subdivide the range of wavelengths into finer bands, which allows the shrimp to perceive more contrasts. On the downside, the filters in the eye absorb a lot of light, rendering a darker image. Oberwinkler suspects the mantis shrimp's colorful habitat, including richly tinted prey and sexual partners, may have had something to do with evolution of their color vision overkill. But exactly why or how is still a mystery. "We have no great ideas about that yet," he admits.
It's a "beautiful" paper, says Jelle Atema, a Boston University expert on crustacean senses. He suspects the mantis shrimp's lifestyle--swimming in shallow waters under the tropical sun--is key to understanding why they evolved their Technicolor vision. Normally, he says, crustaceans live at night or in darker habitats, where the absence of bright light and ultraviolet radiation would make it unnecessary for any digital fine-tuning.