How moonlight affects the circadian clocks of animals

While it is known that the circadian clocks possessed by animals are set by sunlight and darkness, little is known about the role of moonlight in the timing of daily behavior. A growing collection of research confirms that there is a connection between animal behavior and lunar cycles. Now, a study from the University of Vienna provides new insight into how moonlight affects animal reproduction.

“Many species synchronize their physiology and behavior to specific hours. It is commonly assumed that sunlight acts as the main entrainment signal for ∼24-h clocks. However, the moon provides similarly regular time information,” wrote the study authors. 

“We have explored the marine bristle worm Platynereis dumerilii to investigate the role of moonlight in the timing of daily behavior. We uncover that moonlight, besides its role in monthly timing, also schedules the exact hour of nocturnal swarming onset to the nights’ darkest times.”

According to the researchers, the study provides an explanation for the phenomenon that daily clocks from flies to humans can exhibit plastic run-times.

The marine bristle worm Platynereis dumerilii releases its eggs and sperm freely into the ocean. The males and females rise from seagrass to the water’s surface, and the timing of this activity is critical for the survival of the species.

Scientists were already aware that bristle worms release eggs and sperm on just a few days of the month. The new study reveals that the worms also synchronize reproductive activities during very specific hours of each night. 

“We show that moonlight determines when, precisely, during the night the worms start their reproductive behavior, which is always during the darkest portion of the night,” explained study first author Martin Zurl. 

Rather than acting as the direct stimulus for swarming, moonlight changes the circadian clock period length, said the researchers. The time of moonlight changes every day by about 50 minutes, and it is the plasticity of the circadian clock that allows the worms to adjust to these changes. 

In collaboration with the lab of Charlotte Helfrich-Förster at the University of Würzburg, the experts demonstrated that the specific decoding of moonlight extends the daily clocks of other species besides bristle worms. 

“Our work reveals that there is an ecological meaning behind the observation that the circadian clock of an individual can run with different speeds,”  explained study co-author Kristin Tessmar-Raible. 

Humans also exhibit circadian plasticity, noted the researchers. For example, patients with bipolar disorder often shift their behavioral and physiological cycles in correlation with their mood switches.

Ultimately, the study authors hope that their work will help to understand the origin and consequences of biological timer plasticity, as well as its interplay with natural timing cues.

The study is published in the journal Proceedings of the National Academy of Sciences.

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By Chrissy Sexton, Earth.com Staff Writer