Added PubMed Id Many animals exhibit strong seasonal rhythms in their physiology and behavior, but the sensors conveying the environmental input are still unknown. In our study we provide detailed light measurements from the natural habitat of the marine bristle worm Platynereis dumerilii, showing that the ratios between different seasons differ more for day-time UV-A light than for longer wavelength. Mimicking seasonal UV-A differences, without photoperiodic changes, significantly decreased locomotor activities in non-mature worms and equaled the reduction occurring with short photoperiod. This UV-A dependent modulation of locomotion requires cOpsin1, likely signaled via Gi-signalling. cOpsin1 mutants also show a strong reduction of rate limiting enzymes for monogenic amine synthesis, as well as regulation of several neurohormones, including NPY, PDF and Vasotocin, as determined via parallel-reaction monitoring in this dataset. Together, these data provide strong functional evidence that cOpsin1 functions as a seasonal light receptor and suggest that daylengths, as well as UV-A levels signal annual time to animals.