Sharks have well-developed eyes and a large brain area dedicated to processing visual information, but these marine predators cannot distinguish between any colors except, in some cases, shades of green, according to a study that examined 17 shark species, including tiger and bull sharks.
By examining the sharks' retinas, located at the back of their eyes, scientists led by Nathan Scott Hart of the University of Western Australia found that the animals had none or only one type of receptor cell, called a cone, that responds to particular wavelengths of light. The cones relay a message back to the brain and the brain interprets this as color, so certain wavelengths correspond to particular colors.
The scientists did not detect any cones in 10 of the species, and found only a single type of cone in the other seven. Those cones responded to wavelengths of light that correspond with the color green. So these sharks could see shades of green, but not other colors.
By comparison, human retinas have three types of color receptors, and birds have four, one of which allows them to detect ultraviolet light, which is invisible to us.
Cones are one of two main types of photoreceptor cells in the eye. Rods are the others. They are sensitive to light and make it possible to see in low-light conditions by detecting contrast between light and dark. Rods were the most numerous photoreceptors among all of the sharks studied.
The researchers said these findings could have implications for preventing attacks on humans and reducing the toll of by-catch, when sharks are inadvertently killed during fishing operations that target other species. [On the Brink: A Gallery of Wild Sharks]
"Our study shows that contrast against the background, rather than color per se, may be important for object detection by sharks," Hart said. "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."
Sharks are efficient predators, and their evolutionary success is thought to be due in part to an impressive range of sensory systems, including vision.
Hart and colleagues used a technique called microspectrophotometry to identify the visual pigments of cones in the sharks' retinas and measure the wavelength of light they absorbed.
Monochromatic vision like this is unusual on land, but in the ocean, the inability to distinguish colors isn't unusual, the authors wrote online Jan. 7 in the journal Naturwissenschaften – The Science of Nature.
Many aquatic mammals – whales, dolphins and seals – also possess only a single, green-sensitive cone type. It appears that both sharks and marine mammals acquired the same biological trait even though their lineages are not related, the authors commented.
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