A half-mile down into the ocean, out of the reach of sunlight, crabs could be using blue and ultraviolet light to find food and avoid toxins: An ability to detect shorter wavelengths may help them discern the good grub from the bad, according to a recent study. The animals might be using light sensitivity to “sort out the likely toxic corals they’re sitting on – which glow, or bioluminesce, blue-green and green – from the plankton they eat, which glows blue,” said Sönke Johnsen, a biologist at Duke University in Durham, N.C. “Call it color-coding your food.” The collaborators studied three ocean-bottom sites in the Bahamas, taking video and images of the crustaceans’ feeding patterns and the wavelengths at which nearby animals bioluminesced. They captured eight crustaceans found at these and other sites and examined their eyes. In the darkness of the deep sea, crabs perch on corals, which are often toxic, and which bioluminesce in green and blue-green (shown). The crabs’ preferred food, plankton, exhibits blue bioluminescence, as seen on the streak across the top. Seeing the difference in colors may help the animals see their food and avoid poisons. Courtesy of Sönke Johnsen, Duke University. Study leader Tamara Frank of Nova Southeastern University’s Oceanographic Center in Dania Beach, Fla., attached a microelectrode to each of the creatures’ eyes, then flashed different colors and intensities of light at them and recorded their eye response. She discovered that all of the species were extremely sensitive to blue light, and two were extremely sensitive to both blue and UV light. The species sensitive to both blue and UV light used separate light-sensing channels to distinguish the colors. It’s the separate channels that would allow the animals to have a form of color vision, Johnsen said. In a true-color image of the bioluminescence of the coral and plankton captured at the site, the coral glows greenish, and the plankton glows blue, Frank said, adding that the plankton is blurred because it’s drifting by as it hits the coral. The image and a video show crabs sitting on a sea pen, periodically picking something off and putting it in their mouths. That behavior, and the data showing the crabs’ sensitivity to blue and UV light, suggests that the creatures have a basic color code for their food. The findings were reported in the Journal of Experimental Biology. The idea is “still very much in the hypothesis stage, but it’s a good idea,” Johnsen said. To further test this hypothesis, the scientists need to collect more crabs and test their sensitivity to even shorter wavelengths of light. Whether the crabs are acting in a natural way during the investigative proceedings remains to be seen. The potential applications of this knowledge are yet unknown. But just look at the x-ray telescope based on lobster eyes reported a few years back. “Sometimes these discoveries can also lead to novel and useful innovations years later,” Frank said.