Crabs may use ultraviolet 'vision' to feed

Some deep-sea crabs have eyes sensitive to ultraviolet light and may be using bioluminescence to help differentiate food from poisons, U.S. scientists say. The crabs might be using their ultraviolet and blue-light sensitivity to sort out the toxic corals they're sitting on -- which, when they glow, show blue-green and green bioluminescence -- from the plankton they eat, which glow blue, scientists said. Crabs living in the deep-sea zone, where little sunlight penetrates the pitch dark, may still be able to "see" in the ultraviolet spectrum. Some deep-sea crustacean species are extremely sensitive to blue light and a few are extremely sensitive to both blue and ultraviolet light, the researchers from Florida's Nova Southeastern University and Duke University found. Species sensitive to blue and UV light used two separate light-sensing channels to make the distinction between the different colors, giving them a form of color vision, a Nova release reported Thursday. The crabs' sensitivity to blue and UV light suggests they have the ability color code their food, researchers said. The idea is "still very much in the hypothesis stage, but it's a good idea," Duke marine biologist Sonke Johnsen said.