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May 29, 2017

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Researchers say sharks see world in black and white

SYDNEY--The U.S. Navy did tests on colors other than yellow for the life jackets military pilots wear in case they have to eject and splash down in the ocean.

They found that sharks were less likely to be attracted to red or black than what one wag called "yum, yum, yellow."

However, those 1970s tests were a waste of time, researchers in Australia have said, because sharks — along with whales, dolphins and seals — are color-blind. Two or more different types of cone opsins — light-sensitive pigments in the photoreceptors in the cells in the retina — are needed for color vision. Sharks, unlike most other fish, have only one cone opsin present.

Humans have three cone opsins in our eyes, enabling us to pick up blue, green and red light.

Nathan Hart, a neuroecologist at Perth's University of Western Australia, said it was likely that the ancestors of sharks could see in color but evolution had lost them this facility.

"It may be that color isn't useful to them or that they've lost the pigments for another reason," Hart said.

He said mammals have done this, reducing their color vision from four channels (tetrachromacy) to two channels (dichromacy).

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