By Victoria Gill

Science and nature reporter, BBC News

The researchers examined 17 species of shark, including the bull shark

Researchers in Australia have discovered a secret weakness of one of the ocean's most impressive predators. Sharks, it seems, are completely colour blind. The scientists, who examined retinas of 17 different species of shark, discovered that the creatures had only one type of colour-sensitive cell, known as a cone cell, in their eyes. Human eyes have three cone cell types, with each type dedicated to receiving either blue, green or red light. This allows most people to tell the difference between different coloured objects. This may help us to design swimming attire that is less attractive to them

Biologist Dr Nathan Scott Hart The study, carried out by Nathan Scott Hart and colleagues from the University of Western Australia and the University of Queensland, Australia is published in the journal Naturwissenschaften. Dr Hart said the research could help to prevent shark attacks on humans and assist in the development of fishing gear that may reduce the number of sharks that are caught accidentally in long-line fisheries. "Our study shows that contrast against the background, rather than colour per se, may be more important for object detection by sharks," he explained. "This may help us to design long-line fishing lures that are less attractive to sharks as well as to design swimming attire and surf craft that have a lower visual contrast to sharks and, therefore, are less 'attractive' to them." To reveal the secrets of shark vision, the team examined the retinas of 17 shark species caught in a variety of waters in both Queensland and Western Australia. SOURCES

Read the paper on shark colour-blindness in Naturwissenschaften More on sharks from BBC Wildlife Finder In humans, the retina is densely packed with light-sensitive rod and cone cells. Rods are able to function in much lower light than cones - and these were the most common cell types found in all of the sharks. In ten of the species they examined, the researchers found no cone cells at all. In the seven species that did have cones - only a single type, highly sensitive to just one wavelength, and therefore one colour, was present. If sharks' retinas cannot distinguish between different colours of light, as this research suggests, they may be totally colour-blind. Dr Hart explained that, because sharks live underwater and many are most active at dawn and dusk, they are operating in relatively dim light. "Rods are what humans use to see at night," he said. "But some sharks are active at all times of the day and they have more cones and fewer rods than the nocturnal or deeper diving species. Why they are apparently not interested in colour is a mystery and one we hope to find out through further research." It seems clear from these findings that if sharks are "attracted" to an object in the water, this is probably because of its contrast against the surrounding water, rather than it's colour. "Bright yellow is supposed to be attractive to some sharks, presumably because it appears to the sharks as a very bright target against the water," said Dr Hart. "So perhaps it is best to avoid those fluoro-yellow shorts next time you are in the surf."



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