It's time for a rethink about the colors of swimwear and protective clothing as Australian researchers have found that some sharks may be color blind.
Sharks may only be able to distinguish shades of grey, and may depend on contrast to hunt their prey.
Bright colors and highly contrasting colors may stand out in the general grey background of the ocean.
Researchers from the University of Western Australia and the University of Queensland have been examining shark eyes under the microscope.
They used a procedure called micro-spectrophotometry, and found that some shark retinas contain only one type of cone photoreceptor. Humans have three cone types that are sensitive to blue, green and red light. This means that sharks can only see one color.
The study shows that contrast against the background, rather than color itself, may be more important for object detection by sharks.
This has several important implications:
Dr. Nathan Scott Hart stated in a press release that this new research on how sharks see may help to prevent attacks on humans by designing swimming attire and surf craft that have a lower visual contrast to sharks and, therefore, are less 'attractive' to them.
To date, it is unclear whether sharks have color vision, despite having well-developed eyes and a large sensory brain area dedicated to the processing of visual information.
The researchers looked at the retinas of 17 shark species.
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.
While cone monochromacy is rare in land animals, many aquatic mammals - whales, dolphins and seals - also possess only a single, green-sensitive cone type.
The reason for this is unknown, but it is clearly adaptive for marine animals.
The research is to be published in the paper "Microspectrophotometric evidence for cone monochromacy in sharks" Naturwissenschaften Online First™, 6 January 2011
It is interesting to review the statistical data complied in the The International Shark Attack File on the Clothing and Gear Worn or Carried by the Victim of an Unprovoked Attack which looks at the color, contrast and other features of the clothing and gear worn by shark attack victims.
Most people would suggest that glossy or shiny features on apparel or equipment would be more attractive to sharks, but the statistics on shark attacks victims appears to contradict this.
The largest percentage of victims were wearing clothing with no exceptional characteristics or shininess on their apparel.
The next largest percentage of victims had highly contrasting colors on their outfits or gear.
This results mirrors the finding of the research on sharks's eyes that suggests that contrast may be an important element of shark's vision.
With these statistics there is no evidence of cause and effect relationships because the statistic may merely reflect user preferences.
It is likely for instance, that more divers wear highly contrasting colors on their outfits, or wear plain outfits with no special glossy features.
It is likely that user preferences may explain these statistics.
Plain Black is the most common color on the equipment and apparel of divers who were attacked by sharks.
A combination of black and plain blue was the second most common color blend, and plain blue was the third.
These statistics suggest that black and dark blue colors that attract sharks.
However it is highly likely that these differences occur simply because black and dark blue happens to be the most popular color of gear and clothing worn by divers.
Certainly the statistics may need to be reinterpreted in the light of the discovery that some sharks are color blind.
Similarly the use of high contrast protective clothing designed for rescuers to easily spot people lost at sea, may need to be re-examined in terms of reducing the risk of shark attack.