"Numerous scientific studies have established that the lunar cycle synchronizes reproductive behaviour and sexual maturation of animals as diverse as corals, polychaetes and fishes. Classical and recent work shows that in animals such as the annelid Platynereis dumerilii, dim nocturnal light serves as entrainment cue for an endogenous oscillator – a circalunar clock – that orchestrates reproductive and behavioral cycles. As circalunar clocks run with a (semi-)monthly period, they represent a fundamental biological phenomenon clearly distinct from the widely studied, solar light-entrained circadian (24h) clocks. Despite the vital importance of circalunar clocks, very little is known about the underlying molecular processes and responsible neuron types. This knowledge gap reflects the fact that until now, no suitable model system has been available to study circalunar clocks on the molecular and cellular level.This proposal takes full advantage of the recent establishment of substantial molecular resources and critical techniques for functional analyses in Platynereis, as well as our pioneering work on the first circalunar clock-regulated genes and the identification of four molecular candidates for the nocturnal light receptor.This now allows us to tackle two fundamental objectives:First, we aim to discover the molecular and cellular nature of the lunar light sensor(s) and their interplay with solar light photoreceptors.Second, we aim to characterize circalunar oscillatory genes and their associated neuron types thatwill pave the way to unravel the molecular and cellular nature of the circalunar oscillator.This work will provide new mechanistic insight into an unexplored biological mystery- circalunar clocks and their regulation by light. It also offers new conceptual advance into how animals accomplish the separation of diurnal versus nocturnal light information for the synchronization of reproductive behaviour, a challenge common in the natural environment."