How the moonlight tunes the reproduction of animals

Animals have circadian clocks or 24-hour oscillators that regulate daily behavior. They usually take stimuli from periodic changes of sunlight and darkness. However, many animals are also exposed to moonlight, which repeats with a periodicity of ~ 25 hours.

Florian Raible Laboratories at Max Perutz Laboratories, a joint venture of the University of Vienna and the Medical University of Vienna, and Kristin Tessmar-Raible (Max Perutz Laboratories, Alfred Wegener Institute, University of Oldenburg) have now discovered that the moonlight adapts to the day of the sea bristle worms, which help them tune the reproductive cycle to certain hours of the night. Study published in Proceedings of the National Academy of Sciencesprovides an explanation for the phenomenon that daily hours from flies to humans may show plastic running time.

In order to give birth to the next generation, the sea bristle worm Platynereis dumerilii releases its eggs and sperm freely into the open sea water. Proper timing of their reproductive cycles is therefore essential for the survival of the species. It has previously been known that bristles plan to reproduce for several days a month. Now scientists have discovered that they also synchronize with very specific clocks during each night. “We show that moonlight determines exactly when worms begin their reproductive behavior during the night, which is always during the darkest part of the night,” explains first author Martin Zurl. Moonlight does not act as a direct stimulus for swarming, but changes the length of the circadian clock period. In nature, the time of moonlight changes by about 50 minutes every day. The plasticity of the clock allows the worms to account for these changes.

The researchers worked with the laboratories of Robert Lucas at the University of Manchester (UK) and Eva Wolf of the Institute of Molecular Biology Mainz and the University of Mainz (Germany) to characterize the light receptors involved in this process. They found that the combinatorial functions of opsin – closely related to the mammalian circadian photoreceptor melanopsin – and cryptochrome decoded sunlight and moonlight differently to adjust the plastic time of day. In collaboration with the Charlotte Helfrich-Förster Laboratory at the University of Würzburg (Germany), the researchers have shown that specific moonlight decoding is also relevant for daytime clocks of other species.

The effect of light intensity on the length of the circadian clock period has been documented for various organisms in artificial laboratory conditions for decades. However, the physiological relevance of these observations was unclear. “Our work reveals that there is ecological significance behind the observation that an individual’s circadian clock can run at different speeds,” explains Kristin Tessmar-Raible. It should be noted that humans also exhibit such circadian plasticity. For example, patients with bipolar disorder exhibit mysterious circulatory (i.e., 24.8 h) periods correlating with their mood swings. The researchers hope their work will help understand the origins and implications of the plasticity of the biological timer, as well as its interplay with natural time signals.