Moonlight receptor of the '1-h-midge' Clunio marinus studied by micro-XRF

Melanin is a pigment widely occurring in animals, plants, fungi and algae. It does not only colour skin, hair and eyes but serves mainly as photoprotectant and prevents overload with minerals induced by inflammations, infections and degenerative diseases. Therefore, the mechanisms underlying melanisation gained increasing interest in the field of biomedical research and clinic. So far, the processes of melanogenesis are only partly analysed, nearly nothing is known on a putative switch between melanins of different types. Here we offer a model organism to study these mechanisms as part of a naturally cycling change of transparency of the retinal shielding pigment. A marine midge, Clunio marinus, living in coastal regions, underlies a complex timing of its development by solar and lunar climatic periodicities, which synchronise biological clocks. The question was how the animals can discriminate changing sunlight from moonlight intensities. For the first time, we could show a 'moonlight window' in the larval ocelli of this midge, and propose a hypothesis on the underlying mechanisms. Driven by a lunar clock the image forming ocelli become transparent and convert during moonlit nights to a sensitive photometer, which can record the dynamics of environmental light. High resolution X-ray fluorescence (XRF) measurements of the distribution of trace minerals in single melanosomes combined with their fine structural details in various states of the lunar cycle provide a first insight into the enzymatic pathways for the generation of a dark melanin (like eumelanin) and a light coloured melanin (like phaeomelanin). Essential advantage of this approach is the spatial and temporal resolution of the metals associated with melanisation processes, which could never before be demonstrated in these details. The data may stimulate further research projects in biomedicine