Sharks may be able to smell blood from miles away, but they probably don't know how red it is: New research suggests sharks are color-blind.
Sharks have successfully prowled the oceans for millions of years, in part because of an impressive suite of sensory systems, including well-developed eyes and a large area in the brain devoted to vision.
But over the past few decades conflicting data have sparked a debate about whether sharks can see colors. (Related: "Color-Blindness Cured by Gene Injection in Monkeys.")
Retinas use two main types of light-sensitive cells to allow animals to see: Rod cells help measure brightness, while various types of cone cells help distinguish colors.
A technique known as microspectrophotometry had previously shown that rays and chimaeras, both close relatives of sharks, have color vision. (Related pictures: "Weird New Ghostshark Found; Male Has Sex Organ on Head.")
Using the same method, Nathan Scott Hart at the University of Western Australia and colleagues scanned shark retinas for pigments linked with rod and cone cells.
Rod cells were the most common types of photoreceptor found in all 17 shark species, which means the predators should be able to see within a wide range of light levels.
But no cone cells were observed in 10 of the 17 species, while only one type of cone cell appeared to be present in the other 7. This suggests that these sharks cannot tell different colors apart.
"We can't say hands down if sharks are color-blind yet, as there are over 400 different shark species," said shark biologist Michelle McComb at Florida Atlantic University's Harbor Branch Oceanographic Institute.
"But these findings are excellent, and a surprise, and definitely should spur more work," said McComb, who did not take part in the new study.
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It makes sense that sharks might be color-blind, the study team noted. Many aquatic predators—such as whales, dolphins, and seals—also appear to be color-blind, which may be because color vision isn't much use in their mostly blue-green environments.
If the discovery holds true for more shark species, it could be used to help reduce the numbers of endangered sharks accidentally caught by fisheries, as well as prevent shark attacks on humans. (See "Eight Million Sharks Killed Accidentally off Africa Yearly.")
"Our study shows that contrast against the background, rather than color per se, may be more important for object detection by sharks," study co-author Hart said in a statement.
"This may help us to design longline 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."
The shark-vision study was published online January 6 by the journal Naturwissenschaften.