New research on how sharks see may help to prevent attacks on humans and assist in the design of fishing gear that may reduce shark bycatch in long-line fisheries.
The joint study between researchers at The University of Queensland and The University of Western Australia looked at the potential for colour vision in a number of Australian shark species.
The team measured the light-sensitive cells in the sharks' eyes using a specialised instrument called a microspectrophotometer and concluded that they have only one type of cone photoreceptor in the retina.
"Humans have three cone types that are sensitive to blue, green and red light, respectively, and by comparing signals from the different cone types we get the sensation of colour vision," said UWA’s Associate Professor Nathan Hart.
"However, we found that sharks have only a single cone type and by conventional reckoning this means that they don't have colour vision."
UQ’s Dr Susan Theiss said the sensitivity of the sharks’ cone photoreceptors varied slightly between species, which most likely reflected differences in preferred habitat type or behaviours.
“Compared to the other shark species, wobbegong sharks have a red-shifted cone type,” she said.
“These are strongly benthic animals, often found in shallow water, so a red-shifted cone might be an adaptation for detecting and ambushing fish swimming close to the bottom.”
It has long been assumed that sharks have some sort of colour vision and indeed have a preference for certain colours.
The term ‘yum, yum yellow' was coined when it was discovered, in tests by the US Navy, that some species of shark were attracted more to yellow than to other colours.
This was of concern as the Navy wanted to supply its sailors with yellow life vests that would be more visible in rescue situations.
"Our study shows that contrast against the background, rather than colour per se, may be more important for object detection by sharks, and this may help us to design long-line fishing lures that are less attractive to sharks — while still effective for the target fish species — and thus help to reduce the massive by-catch of sharks in this industry."
One of the species the team studied, the bull shark Carcharhinus leucas, is responsible for numerous attacks on humans. The bull shark is an aggressive species and often inhabits shallow, murky water close to human habitations.
"Now we know a bit more about how such sharks see the world, it may be possible to design swimming attire and surf craft that have a lower visual contrast to sharks and, therefore, are less ‘attractive' to them,” Dr Hart said.
“After all, most shark attacks are thought to be the result of curiosity on the part of a shark that has been attracted to an unusual stimulus, rather than some premeditated ambush."
The findings of the research are described in the paper Microspectrophotometric evidence for cone monochromacy in sharks, co-authored by Dr Theiss and UQ’s Blake Harahush, with WA Premier's Research Fellow Winthrop Professor Shaun Collin, to be published in the international journal Naturwissenschaften - The Science of Nature.
Media: Dr Susan Theiss, UQ, ph +61 7 3365 9758
Fiona Cameron, UQ Communications, ph +61 7 3346 7086
Associate Professor Nathan Hart, UWA, +61 8 6488 3960
Janine McDonald, UWA Public Affairs, +61 8 6488 5563
