The sun exerts hegemony over biological rhythms of nearly every organism on Earth. But two studies now show the moon is no slouch. It controls the cadence of at least two different biological clocks: one set by tides and the other by moonlight.
The clocks, both discovered in sea creatures, work independently of the circadian clock, which synchronizes daily rhythms with the sun. The studies demonstrate that the moon’s light and its gravitational pull, which creates tides, can affect the behavior of animals.
“The moon has an influence, definitely,” says Steven Reppert, a neurobiologist at the University of Massachusetts Medical School in Worcester, who was not involved with either study. “Clearly for these marine organisms, it’s very powerful and important.”
Scientists established decades ago that circadian clocks govern people’s daily cycles of such things as hormone levels, blood pressure and body temperature. Nearly every organism, including single-celled creatures, has some version. Circadian clocks are composed of protein gears. In a loop that takes roughly 24 hours, levels of some proteins rise and then fall, while others fall and then rise. Sunlight sets the clocks, but once a clock is set it will keep running, even when scientists keep organisms in constant darkness.
Other rhythmic behaviors occur on longer time frames, such as reproductive cycles that seem to follow the moon, annual patterns like hibernation and blooming cycles, and multiyear events like the emergence of cicadas every 13 to 17 years. Other periodic activities happen on shorter timescales, such as behavior of coastal organisms coordinated with tides. Researchers have debated whether these behaviors were really timed by an internal clock that would keep ticking if the cues used to synchronize it disappear.
“What is biologically true and what is myth needs to be carefully untangled,” says Kristin Tessmar-Raible, a molecular neurobiologist at the University of Vienna. She and colleagues describe a lunar clock in a marine worm in the Oct. 17 Cell Reports.
From astronomy to zoology
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That unraveling of fact from fiction can take a long time. It took about nine years for Charalambos Kyriacou of the University of Leicester in England and his colleagues to establish that the speckled sea louse, Eurydice pulchra, has a clock that times the tides. Before the tide goes out, the creatures bury themselves in the sand to keep from being swept out to sea. When water levels rise, 12.4 hours later, the sea lice emerge to forage. When kept in dark, still water in the lab, the animals’ swimming patterns still follow the rise and fall of the tides, indicating that the rhythm is under control of a tidal clock within the sea lice, the researchers report in the Oct. 7 Current Biology.
The scientists’ work has taken on a rhythm, too, one dictated by an organism that lives for only a few months and doesn’t breed in the lab. Each spring and summer, the researchers fish the marine crustaceans out of high tides. Winters are spent analyzing the animals’ behavioral data and genetic material.
Some researchers had speculated that Eurydice’s tidal rhythms might stem from a pair of out-of-phase circadian clocks, generating the roughly 12-hour tidal rhythms. Others thought an independent clock could drive tidal rhythms.
So the researchers disabled genes that make two of the molecular gears in the crustaceans’ circadian clock. “It doesn’t matter what you throw at the circadian clock — you can hit it with a hammer — and the tidal rhythm is unaffected,” Kyriacou says. That is evidence that the tidal clock uses different protein gears than the circadian clock does.
As Kyriacou’s group prepared to publish its results, Tessmar-Raible and her colleagues were simultaneously reporting their discovery of a lunar clock in a marine worm, Platynereis dumerilii. The worms spawn on a monthly cycle set by moonlight, the team found.
Like the tidal clock in the crustaceans, the worms’ lunar clocks kept on ticking when the researchers dismantled the circadian clock. That finding indicated the monthly cycles are under control of an independent timing mechanism.
The discoveries raise the possibility that many other organisms, including humans, may have multiple timers, says Charlotte Helfrich-Förster, who studies biological clocks at the University of Würzburg in Germany. Such clocks could be behind women’s monthly menstrual cycles; recent studies have also shown that sodium levels have a monthly rhythm and that people’s sleeping habits may follow lunar cycles (SN: 08/24/13, p. 15).
The researchers are now trying to find the molecular gears that run the new clocks.