Are sharks color blind?

Sharks are unable to distinguish colors, even though their close relatives rays and chimaeras have some color vision, according to new research by Dr. Nathan Scott Hart and colleagues from the University of Western Australia and the University of Queensland in Australia. Their study shows that although the eyes of sharks function over a wide range of light levels, they only have a single long-wavelength-sensitive cone* type in the retina and therefore are potentially totally color blind. Hart and team's findings are published online in Springer's journal Naturwissenschaften – The Science of Nature.

"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 colour 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 Prof. Hart.

Sharks are efficient predators and their evolutionary success is thought to be due in part to an impressive range of sensory systems, including vision. To date, it is unclear whether sharks have color vision, despite well-developed eyes and a large sensory brain area dedicated to the processing of visual information. In an attempt to demonstrate whether or not sharks have color vision, Hart and colleagues used a different technique - microspectrophotometry - to identify cone visual pigments in shark retinas and measure their spectral absorbance.

They looked at the retinas of 17 shark species caught in a variety of waters in both Queensland and Western Australia. Rod cells were the most common type of photoreceptor in all species. In ten of the 17 species, no cone cells were observed. However, cones were found in the retinae of 7 species of shark from three different families and in each case only a single type of long-wavelength-sensitive cone photoreceptor was present. Hart and team's results provide strong evidence that sharks possess only a single cone type, suggesting that sharks may be cone monochromats, and therefore potentially totally 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."

*There are two main types of photoreceptor cells in the retina of the eye. Rod cells are very sensitive to light and allow night vision. Cone cells also react to light but are less sensitive to it. Eyes with different spectral types of cone cells can distinguish different colors. Rod cells cannot tell colors apart.

Reference
Hart NS et al (2011). Microspectrophotometric evidence for cone monochromacy in sharks. Naturwissenschaften – The Science of Nature; DOI 10.1007/s00114-010-0758-8

The full-text article is available to journalists on request.