Fish may use ultra violet (UV) light to set their body clock, according to a new study published in the Proceedings of the National Academy of Science (PNAS).
Dr Akihasa Terakita and co-workers from Japan's University of Kyoto studied the pineal gland of the Japanese river lamprey, Lethenteron japonica. This light detecting organ contains a pigment called parapinopsin which is sensitive to UV light.
The study found that parapinopsin could switch between two stable forms, when exposed to either UV or visible light. UV exposure caused the pigment to absorb green light, while exposure to normal light caused the pigment to return to its normal state.
The scientists believe that the fish may use this pigment in circadian regulation, rather than visible light.