Sharks see world as 50 shades of grey

Sharks are colour blind, a new molecular study by Australian scientists has confirmed, filling a gap in our knowledge about the evolution of colour vision. Dr Susan Theiss, from the University of Queensland, and colleagues, report their findings in the journal Biology Letters.

The evolution of colour vision has been studied in most vertebrates, but until recently, elasmobranchs (sharks, skates and rays) had been overlooked. Previous physiological research has shown some rays have colour vision but it suggested sharks were colour blind.

These previous studies looked at opsins, which are light-sensitive proteins found in the photoreceptor cells of the retina. Rod opsins are used in low light and produce a black and white image, while cone opsins are used in bright light, and often to see colours. Two or more different types of cone opsins are needed for colour vision.

While some ray species have multiple cone opsins as well as rods, studies in various shark species suggested they had only a single cone visual pigment.

To check whether this really was the case, Theiss and colleagues isolated the visual opsin genes from two wobbegong shark species: the spotted wobbegong Orectolobus maculatus and the ornate wobbegong O. ornatus.

Their findings confirm that wobbegongs possess only one cone opsin, meaning they see the world in shades of grey. The findings help fill in the picture of how colour vision evolved in different species.

"We know the earliest vertebrates had colour vision, but it has been lost by some groups over the course of evolution," says co-author Associate Professor Nathan Hart, a neuroecologist at the University of Western Australia.