According to new research, sharks, unlike their close relatives chimaeras and rays, cannot distinguish colors.
This study was performed by Dr. Nathan Scott Hart and colleagues from the University of Western Australia and the University of Queensland in Australia.
Sharks have a large sensory brain area dedicated to the processing of visual information and they have very well-developed eyes but it is still unclear whether they have color vision. They are top predators, due in part to their extensive range of sensory systems, including vision. Their eyes function over a wide range of light levels but their research shows that they are potentially completely color blind because they only have a single long-wavelength- sensitive cone type in the retina.
There are two main types of photoreceptor cell in the retina. Rods are sensitive to light and allow night vision while cones also react to light but are much less sensitive. Eyes with different types of cones can distinguish colors. Rods do not have this ability.
“This new research on how sharks see may help to prevent attacks on humans and assist in the development of fishing gear that may reduce shark bycatch in long-line fisheries. Our study shows that contrast against the background, rather than color per se, may be more important for object detection by sharks. 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,” said Professor Hart.
In their attempt to determine if sharks have color vision, Hart and his team used microspectrophotometry to identify cone visual pigments in shark retinas and measure their pectral absorbance. They looked at the retinas of 17 shark species caught in Queensland and Western Australia. Rod cells were the most common photoreceptor found in all species. In ten of the 17 species, no cone cells were observed at all. In the seven species where cone cells were found, only a single type of cone photoreceptor was present. These sharks were from three different families.
These results provide strong new evidence that sharks may be completely color blind. The authors conclude: “While cone monochromacy on land is rare, it may be a common strategy in the marine environment. Many aquatic mammals − whales, dolphins and seals − also possess only a single, green-sensitive cone type. It appears that both sharks and marine mammals may have arrived at the same visual design by convergent evolution, in other words, they acquired the same biological trait in unrelated lineages.”
Copyright © 2010 by Marine Science Today, a publication of OceanLines LLC.