Paris: Sharks may be unable to distinguish
between colours, according to a lab study published on Tuesday that
could benefit swimmers, surfers and sharks themselves.
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 colours 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
colours.
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 colours, say
the investigators.
Monochromatic vision is very rare among land species,
because colour vision is a tool for survival in terrestrial
habitats.
But it is less important in the marine environment, where
colours 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.
The study, published in English in the German journal
Naturwissenschaften, could help prevent shark attacks on
humans and develop fishing gear that could reduce accidental
catches of sharks by long-line trawlers.
"Our study shows that contrast against the background,
rather than colour per se, may be more important for object
detection by sharks," said lead scientist Nathan Scott Hart at
the University of Western Australia.
"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."
PTI
