Crab Species Thought to Use Color to Discern Food Sonke Johnson Some crabs living on the dark sea floor are sensitive to ultraviolet light. Two species of crabs that live on the seafloor have color-coding abilities that help them detect food, a new study suggests. Most deep-sea creatures do not see in color, but the researchers say that these crabs are sensitive to ultraviolet light, which helps them distinguish between blue and green light. The crabs sit atop coral, looking for plankton to feed on. Both the coral and the plankton are bioluminescent. But the coral could be toxic to the crabs, so somehow they have to tell the difference. "This little crab is sitting in the dark, and every time plankton hits the coral it glows blue," said Sonke Johnsen, a sensory biologist at Duke University and the lead author of the study, which appears in the journal Experimental Biology. "But the corals also glow, in green," he continued. So what you really need is color vision - you want to be able to color-code your food. He added that the researchers had not yet "confirmed it behaviorally," but that the evidence for the color-coding behavior was strong. The researchers studied crabs in three ocean-bottom sites near the Bahamas. They gathered video and images that helped them piece together what the animals eat. They also captured and examined eight crabs and found that the eyes were sensitive to ultraviolet light. The next step is to try to study the crabs in a laboratory setting, Dr. Johnsen said.