Fig. 7: The entrainment of the monthly oscillator requires the detection of a specific moon phase.

a Representation of the presence of the moon on the sky depending on moon phase. As a full cycle of the moon around the earth takes on average 24.8 h the presence of the moon relative to the sun shifts every night by ~49 min, indicated by the green diagonal. Worms need to specifically detect the full moon phase for circalunar oscillator entrainment, which requires that they can realize when a specific light (moonlight) starts and ends²⁶. b L-Cry’s function as moonlight duration and intensity detector for circalunar clock entrainment. L-Cry’s biochemical property to only reach the full moonlight state after extended periods of (naturalistic) moonlight illumination allows for a discrimination of moonlight duration. As moon phases are characterized by the duration (and intensity) of the moon on the night sky, moonlight exposure duration translates into moon phase detection. When L-Cry is in its moonlight state it permits (thick green arrow) efficient entrainment of the circalunar oscillator via a yet unidentified photoreceptor X, while L-Cry (partly) blocks (dotted line) the light for circalunar entrainment when it is in its sunlight state. L-Cry might also provide minor information as entraining photoreceptor to the circalunar oscillator (thin green arrow).