Ever since the dawn of life, ecosystems are governed by periodic changes in light conditions that act as reliable cues to synchronize biotic processes. Most, if not all organisms feed this light information into molecular clockworks that allow them to anticipate rhythmic changes in their environment and to synchronize specific biological events.

For decades, efforts to study the underlying molecular mechanisms have focused almost exclusively on land model species and their most prominent light-dark cycle: the circadian rhythm. A key finding of this work was that in eukaryotes the underlying common core mechanism consists of negative transcriptional/translational feedback loops formed by a set of regulatory genes ('clock').

However, life evolved in the sea, and a rich body of literature describes rhythmic phenomena in marine organisms that are unparalleled on land. These exceed circadian rhythms and include tidal, lunar and semilunar rhythms.  Yet, albeit important and widespread (publ. 1, Fig. 1), not a single molecule has clearly been implicated in a moon-entrained clock, due to the lack of a suitable molecular model species.

We aim to fill this gap by analyzing the molecules that govern the lunar reproductive periodicity of the marine bristle worm Platynereis dumerilii (Fig.2). Its large sequence resources and molecular tools, along with its ancestral-type nervous system make it an ideal starting point not only to unravel the molecular principles of its circalunar and circadian clocks, but also to place both clocks in an evolutionary context.

Using these worms, we have started to address the following three main questions:

• How are lunar and circadian clock interconnected?
• What is the nature of the lunar clock in Platynereis?
• What are the moon light sensors on the cellular and molecular level?

Platynereis possesses multiple Opsin-based photoreceptor cells, some of which are part of the eyes, while others are located in the medial region of its forebrain. We have started to especially focus on the latter as candidate moon light receptors.

Interestingly, such inner brain photoreceptor cells also exist in the forebrain of vertebrates. In fact, we could show that these cells belong to the most ancient cell types present in the vertebrate brain (see selected publ 3). Yet, we know nothing about their function. We have also started to solve this riddle, using the zebrafish and medakafish. We approach the function of these Opsins using biochemical analyses, zinc- finger mediated mutagenesis, transgenesis, whole mount in situ hybridization, immunocytochemistry, as well as electron microscopy.