The impact of lunar cycles on human behavior and biological processes, including sleep habits, mental illness, and menstrual cycles, has been debated for decades.1,2 For many marine organisms, however, the biological significance of lunar rhythms is more clear-cut: Some corals, crabs, and puffer fish use the phases of the moon to coordinate the mass spawning events necessary to produce the next generation.3
The underlying biology that synchronizes these animals to lunar rhythms remains largely a mystery. Romantic activities in the lab of chronobiologist Kristin Tessmar-Raible at the University of Vienna flatwormA small, orange, nautical bristle bug helps shed light on this phenomenon.
Tessmar-Raible says that for organisms like the bridleworm that rely on in vitro fertilization (where eggs and sperm meet outside the body), reproductive success is maximized when each releases their gametes into a body of water at nearly the same time. Therefore, the worm needs to “schedule” the time and date of the event. For this particular species, spawning events are more likely to occur a few days after a full moon.
Decades of research have shown that circadian rhythms are determined by internal molecular clocks that respond to external signals, such as light. Tesma-Rebull showed that the worms’ lunar clocks function in a similar way: After experiencing an artificial lunar cycle for two months, the worms maintained a proper reproductive rhythm for several months without nighttime light, This suggests they have a faithful internal clock.4 On the other hand, worms that never received the correct moonlight cues were unable to develop these reproductive rhythms. “They use the light of the full moon to synchronize their internal calendars,” Tesma Reible said.
To synchronize with the lunar cycle, the worms need a way to differentiate between the full moon’s light and that of the sun. The researchers discovered a worm photoreceptor protein called L-Cryptochrome, or L-Cry, that exhibits different biochemical reactions and localizes to different areas within the cell depending on the intensity and duration of light.5 Tessmar-Raible and her colleagues hypothesized that L-Cry functions like a kind of gatekeeper, determining which light is most effective at resetting the worm’s internal clock.
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