Beneath the surface of the ocean, a secret choreography unfolds—timed not by clocks or calendars, but by the silver light of the Moon. Coral reefs, sea worms, and even tiny sand-dwelling crustaceans all keep time with lunar precision, releasing eggs, changing color, and rising to mate in breathtaking synchrony. The Moon, it turns out, isn’t just a passive backdrop in the night sky—it’s a living metronome for life on Earth.

In “Our Moon: How Earth’s Celestial Companion Transformed the Planet, Guided Evolution, and Made Us Who We Are,” science journalist Rebecca Boyle expands our view far beyond tides and phases. She explores the Moon’s role in everything from the evolution of complex life to ancient religion, scientific discovery, and even future space politics. The result is a sweeping, deeply researched, and utterly enchanting account of how our oldest companion has quietly shaped nearly every facet of life.

In the excerpt below, Boyle explores the Moon’s surprising influence on marine biology and animal behavior—where its light continues to guide the rhythms of reproduction, survival, and timekeeping, even in the absence of eyes.

This excerpt has been reprinted with permission from “Our Moon: How Earth’s Celestial Companion Transformed the Planet, Guided Evolution, and Made Us Who We Are” by Rebecca Boyle, published by Random House, 2024. 

On reefs around the world, from the Great Barrier Reef to the middle of the Red Sea, corals time their mating dance according to the full Moon’s appearance. Only after the full Moon has shone upon them will they release their pearlescent sperm and eggs, in a midnight phantasmagoria that biologist Oren Levy describes as “the greatest orgy on Earth.”

Levy is an Israeli coral researcher who grows corals in tanks in his lab at Bar-Ilan University to study their spawning behavior and how it changes in response to light pollution, which interferes with the Moon’s beacon. When he is not raising corals by hand, Levy snorkels to a reef in the Red Sea, near the Israeli resort town of Eilat on the Israel-Jordan border. It is the world’s northernmost tropical reef, and corals there have been exposed to development, pollution, and artificial light for millennia. And yet they still use the Moon as their guide.

“We are talking about an organism that doesn’t have any eyes. And it can still synchronize this behavior to the Moon’s cycles,” he told me.

The corals, which are tiny animals, produce parcels that wait like deliveries on a doorstep, near the threshold where their tiny bodies meet the sea. Then in an instant, in one of the most stunningly synchronous events on this planet, every coral releases its sperm and eggs. All at once, a pink blizzard of uncountable seeds floats up toward the light of the Moon. Many seeds will end their journeys as food for fish and other larger animals. But some coral sperm and eggs will combine, producing new coral larvae, which will bob with the tide until they can find a hard surface to anchor on and build a new city.

The seas’ temperature, wind, and sunlight intensity set the month of spawning. But the Moon and its light set the day and the hour. Corals must release their packets at the same time to have any chance of forming new corals. This Moon-mediated mating dance may be more important than ever as corals worldwide succumb to mass bleaching events and other ravages of a changing climate. New generations of corals will need the Moon to colonize the reefs built by their ancestors. The next time you walk outside under a full Moon, witnessing the milky glow it casts over the trees and the grass and the buildings, think about what is happening, that very night, within this planet’s oceans. How many organisms are being born, guided by the light of the silver pendulum in the sky.

For many organisms the Moon is a vital “zeitgeber” (a word borrowed from German because there was no English equivalent that means something like “time giver”) just as much as the Sun is. But only very recently have chronobiologists started to unravel how this works. The ability to tell time by the Moon has genetic underpinnings, which probably date to the origin of genes, which is to say the origin of life.

In 2013, chronobiologists found in a marine worm and a sea louse the first evidence for a genetic “Moon clock” as distinct from a circadian clock. The speckled sea louse, Eurydice pulchra, is a tiny crustacean one-third of an inch across and in the same taxon as crabs and lobsters. It lives in the intertidal zone and burrows into the sand when the tide goes out, turning itself black for protection against the Sun. It does this using chromatophores, a special type of cell that contains color; the same cells allow cephalopods like octopuses to produce camouflage. The sea lice can sense light, including the spectral illumination provided by the Moon, just as readily as your eyes can. The lice are known to have two types of internal schedules, one governed by the Sun and one that is apparently linked to the tide. But until recently, scientists were unsure whether the tidal clock was derived from the circadian clock—simply by cutting it in half, for instance, with a tidal timekeeper that runs every 12.4 hours. But it turns out the lice have a distinct tidal clock, which runs separately from any Sun-derived rhythm. It is far more complex than simply slicing the day in two.

Scientists in the United Kingdom collected lice from their home beach on a Welsh island, and measured their activity in a tank where they were exposed to either constant darkness or constant light. The researchers then focused on knocking out or suppressing genes known to play a role in the circadian rhythm. The creatures still swam every 12.4 hours, for several days in a row. Suppressing the circadian rhythm did not shut off the lunar rhythm, showing it is an independent system. In the next few years, marine biologists found similar molecular tidal clocks in animals like oysters, curly crustaceans called comma shrimp, and a mangrove cricket.

Using new sequencing techniques, biologists like Kristin Tessmar-Raible are beginning to understand how animals are pulling this off. Tessmar-Raible studies a marine bristle worm, Platynereis dumerilii, which might have one of the most advanced lunar clocks studied so far. The worms follow the Sun for their feeding rhythms, emerging at night to eat. But their spawning cycle follows the Moon alone. The worms use two methods to modulate this lunar clock. They have light-sensitive neurons in their brains, as well as a set of clock genes, related to the same genes found in vertebrates, including you. But the worms’ genetic clock also runs on Moon time.

“When we make an appointment, we don’t tell someone just what time it is; we also, hopefully, give them the date. We intermingle two timing systems, and that is basically what these organisms are also doing,” Tessmar-Raible told me. “There is no voodoo behind it. We have an inner circadian clock. Why shouldn’t animals, or other organisms, also have a calendar system?”

Early in her career, Tessmar-Raible read about previous German biologists who studied these biological rhythms and recalls being shocked that anyone could detect a lunar cadence to an animal’s daily life, and moreover, that it could be controlled in a lab. Then she attended a marine ecology conference and mentioned it to some marine biologists who just looked at her, she recalled. “They were like, ‘Yes, of course. You don’t know about the famous coral example? This happens everywhere.’”

Ancient people knew it, too, though for different reasons. Aristotle knew the sea urchins would swell with the Moon because fishermen had learned that mussels, urchins, and some crustaceans are larger—and worth more money—when their gonads are swollen, ready for reproduction. If you cut open a fresh-caught crab, its reproductive organs will be larger or smaller depending on the Moon’s phase.

The animals have their own practical reasons for this calendar keeping. A marine gnat called Clunio marinus lives along the Atlantic coast of Europe, where scientists have studied its chronobiology for many years. The gnats mate like other insects, with males fertilizing eggs the females laid previously. Because the gnats must wait for an extremely low tide to keep their eggs safe, they evolved the ability to notice the Moon’s phases so they could predict when the tide would flow and ebb. Females will lay their eggs in the lowest levels of the intertidal zone when the tide is at its feeblest, during new Moon or full Moon. Corals do this, too. Levy and his colleagues found that corals have light-sensing neurons that allow them to perceive moonlight on the water. They even have genes that activate in sync with the Moon’s cycles of waxing and waning.

This excerpt has been reprinted with permission from “Our Moon: How Earth’s Celestial Companion Transformed the Planet, Guided Evolution, and Made Us Who We Are” by Rebecca Boyle, published by Random House, 2024.