A ciliary opsin in the brain of a marine annelid zooplankton is ultraviolet-sensitive, and the sensitivity is tuned by a single amino acid residue.

Ciliary opsins were classically thought to function only in vertebrates for vision, but they have also been identified recently in invertebrates for non-visual photoreception. Larvae of the annelid Platynereis dumerilii are used as a zooplankton model, and this zooplankton species possesses a "vertebrate-type" ciliary opsin (named c-opsin) in the brain. Platynereis c-opsin is suggested to relay light signals for melatonin production and circadian behaviors. Thus, the spectral and biochemical characteristics of this c-opsin would be directly related to non-visual photoreception in this zooplankton model. Here we demonstrate that the c-opsin can sense UV to activate intracellular signaling cascades and that it can directly bind exogenous all-trans-retinal. These results suggest that this c-opsin regulates circadian signaling in a UV-dependent manner and that it does not require a supply of 11-cis-retinal for photoreception. Avoidance of damaging UV irradiation is a major cause of large-scale daily zooplankton movement, and the observed capability of the c-opsin to transmit UV signals and bind all-trans-retinal is ideally suited for sensing UV radiation in the brain, which presumably lacks enzymes producing 11-cis-retinal. Mutagenesis analyses indicated that a unique amino acid residue (Lys-94) is responsible for c-opsin-mediated UV sensing in the Platynereis brain. We therefore propose that acquisition of the lysine residue in the c-opsin would be a critical event in the evolution of Platynereis to enable detection of ambient UV light. In summary, our findings indicate that the c-opsin possesses spectral and biochemical properties suitable for UV sensing by the zooplankton model.