(Molecular Biology and Evolution (Oxford University Press)) Until now, little has been known about the evolution of vision in cartilaginous fishes, particularly sharks and their genetic cousins, the rays. In a new study, it has been shown that all cartilaginous fishes, similar to the marine mammals, have lost the SWS1 and SWS2 opsin genes. Sharks and rays do contain both rod and cone photoreceptors; however rays possess two cone opsin genes whereas sharks have only one cone. Sharks therefore were found to have lost the ability to see colors.
Original source: https://www.eurekalert.org/pub_releases/2020-01/mbae-sar010720.php