Sharks Are Color-Blind
The ancestors of sharks might have once seen in color.Corbis Images
Sharks are color blind, new research suggests, with the toothy predators likely forever seeing the world in black and white.
The study, published in the latest Royal Society Biology Letters, is the first to investigate the genetic basis and spectral tuning of the shark visual system.
The ramifications could be huge, helping to save both sharks and people.
"The work will have a major influence on human interactions with sharks," co-author Nathan Hart, a research associate professor at the University of Western Australia's School of Animal Biology and The Oceans Institute, told Discovery News.
"Firstly, this knowledge may enable us to design fishing gear that is more specific for target fish species and thus reduces unnecessary bycatch of sharks," Hart continued. "Secondly, it may help us to design equipment that is less attractive to sharks (wetsuits and surfboards, for example) that may help to reduce attacks on humans."
Building on a study from last year, Hart and his colleagues isolated and sequenced genes encoding shark photopigments involved in vision. Photopigments are light-sensitive molecules. Through a biochemical process, they signal this detection of light to the rest of the visual system.
Photopigments are found in two places: rods and cones. The former type is more sensitive and is generally used under very dim light. The latter type is smaller and less sensitive, but is faster responding, applying more to brighter-light conditions.
The researchers determined that the studied sharks, in this case two wobbegong species, are cone monochromats. This means that the sharks only had one type of cone and one type of rod gene, supporting that they are color-blind. The findings strengthen earlier speculation about not only wobbegongs, but other shark species.
Sharks belong to a cartilaginous fish group that also includes skates and rays. Prior research indicates that skates have "no color vision at all," Hart noted. "Rays have more than one photopigment and so they have the retinal 'machinery' for color vision," he added. "Recent behavioral tests in my lab have also demonstrated that they have functional color vision."
Sharks are probably not the only large water dwellers that are color-blind. Other research indicates that marine mammals, such as whales, dolphins and seals, cannot detect colors either.
"It may be that color is not useful to them, or that they have lost the pigments for another reason," said Hart.
"It is likely that the ancestors of modern sharks could see in color," he added, so sharks and all of these animals may have once seen in color.
Genetic studies even suggest that the ancestors of humans and other terrestrial mammals lost some color sensitivity over the course of their evolution.
Sharks and marine mammals are far from being the most visually challenged aquatic animals.
Recently, scientists studied two groups of blind cave fishes that are eyeless. Prosanta Chakrabarty, an assistant professor and curator of fishes at Louisiana State University's Museum of Natural Science, and colleagues found that such fishes from Madagascar and Australia are related. Life in a dark cave doesn't require color detection or even vision, so the fishes have survived by using their other senses.
Color must also not be critical to shark survival.
"Color as we think of it may be unimportant to sharks, and they are only interested in achromatic contrast differences, just as if we were watching something on a black and white TV set," Hart said.