The eyes of a marine dweller known as the mantis shrimp make use of a biological "sunscreen" that lets it see its underwater surroundings in wavelengths far beyond what the human eye can manage, researchers say.
Their eyes, among the most intricate of any on Earth, can detect a spectrum of colors in both the visible range and far into the ultraviolet, scientists say.
While the human eye can detect color information in visible light by way of three color receptors, mantis shrimp, or stomatopod crustaceans, have 12 receptors, and their complex eyes have built-in UV filters created from the same biological sunscreen other marine animals use to shield their bodies from potentially harmful UV rays.
Six of their receptors, thanks to those filters, are tuned to frequencies completely beyond the visual range detectable by human eyes, says researcher Michael Bok of the University of Maryland.
"The UV filters block certain wavelengths of light from reaching the photoreceptors, chromatically shifting their sensitivity," he says.
"The effect is akin to putting red-tinted glasses over your eyes that block other wavelengths of light, except this is being done at the photoreceptor cellular level in shrimp."
It is unknown exactly why mantis shrimp require a visual system of such sophistication, Bok says.
It may be the abundance of photoreceptors helps the shrimp make sense of complex visual signals coming from its external environment without the need of a large brain to process all the data, he says.
"The way their eyes are built and how visual information is processed in their brains is so fundamentally different from humans that it is very difficult to conceptualize what the world actually looks like to them," says Bok, whose study "Biological Sunscreens Tune Polychromatic Ultraviolet Vision in Mantis Shrimp" has been published in the journal Current Biology.
It is possible the extended visual capability helps the mantis shrimp spot hard-to-find prey in its coral reef home, since many of them absorb ultraviolet light -- and would thus show up as easy-to-see dark objects in an otherwise bright underwater environment.
Mantis shrimp, which can grow to 12 inches long, are found on coral reefs in shallow water in tropical and subtropical oceans where they typically hide in rock formations, waiting for prey.
Their complex eyes are at the ends of long eyestalks that can be moved and aimed independently.
It may also help in the complex social behaviors observed in the shrimp, which sport fluorescent patterns on their bodies which may appear different at various wavelengths.
"It is likely that UV sensitivity plays a role in many of their visually guided behaviors," researchers said (PDF). "Recent behavioral experiments have suggested that stomatopods may use their finely tuned color receptors as a temporally rapid means of color recognition. Provocatively, signaling at exceptionally short UV wavelengths could provide stomatopods with a rapid and covert medium of communication, outside the light-analysis range of any other marine animal."
