Larvae of a marine ragworm Platynereis dumerilii have been studied as a zooplankton model, and possess photoreceptor cells in the brain to regulate circadian swimming behavior. This study revealed that a photoreceptive protein in the brain photoreceptor cells is UV (ultra-violet) sensitive. Since avoidance of UV irradiation is a major cause of a large-scale daily movement of zooplankton, the UV sensor in the brain would be important for physiology and ecology of the zooplankton model.