In order not to poison the food inedible, sea crabs check its quality by color: blue if - then you can eat.
Marine crabs use UV to distinguish edible from inedible report researchers from Duke University and the New Southeast University (both - USA), published on this topic just two articles in the Journal of Experimental Biology (1 and 2). Among the deep-sea animals ability to sense ultraviolet light waves are not so rare, despite the fact that before the light reaches these depths with great difficulty. But until now, zoologists working only with those creatures that live in the water column. But crabs crawling on the bottom of the first hit in their field of vision.
Researchers studied crustaceans living in the Bahamas at a depth of about eight meters. Scientists compared the eating behavior of crabs to the nature of bioluminescence surrounding organisms and the ability to respond to the crab eyes light waves of different lengths. All deep-sea species caught by zoologists proved to be particularly sensitive to blue light. But could distinguish two kinds of usually see blue and ultraviolet. In this case, the difference was seen between the bioluminescent glow of corals and plankton: corals glow green and plankton - blue. Comparing all this with the behavior of feeding crabs, zoologists have come to the conclusion that a special sensitivity to blue and UV is important for crustaceans gastronomic value.
Neighboring corals with crabs may be inedible, toxic. To distinguish somehow fit from the unfit, crabs using a color code that does not allow them to make a mistake.
Researchers themselves, however, specify that their conclusions are based largely on circumstantial evidence, so some questions remain. Why, for example, some crabs need to expend to ultraviolet separate signal channels, why they were not limited to just hypersensitivity to visible blue light? In addition, zoologists do not exclude that the behavior of the crabs might affect the intervention of researchers in their "private life". In short, for the final confirmation of the hypothesis about why crabs ultraviolet vision, need new observations on a variety of crabs and in conditions as close to natural.