Researchers in Australia, using a technique called micro-spectrophotometry, looked at the retinal cells of 17 species of shark caught off Queensland and Western Australia.
In all 17 species, the commonest kind of light receptors were "rod" cells, which are highly sensitive to light and allow night vision but cannot tell colors apart, they found.
Yet the sharks lacked cone cells, which respond individually to light at specific wavelengths. In human eyes, a variety of cone cells helps us to distinguish between colors.
In 10 of the 17 shark species, no cone cells were found at all. Cone cells were found in the other seven species, but they were all of a single type, sensitive to wavelengths of around 530 nanometres, which is green.
This retinal system means sharks are able to tell between shades of grey but, most probably, not between colors, say the investigators.
Monochromatic vision is very rare among land species, because color vision is a tool for survival in terrestrial habitats.
But it is less important in the marine environment, where colors are progressively filtered out at depth and survival depends on distinguishing contrasts, to determine whether a shape in the gloom is prey or predator.
Previous research has found that whales, dolphins and seals also possess green-sensitive cone cells, which suggests that these marine mammals and sharks arrived at the same visual design in parallel, says the paper.
Read more at Discovery News
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