Everyone knows the moon moves the tides. Twice a day the sea rises and falls along the world's coasts, pulled by the moon's gravity. But tides are only the most visible part of a much deeper relationship. Across the oceans, countless creatures time their most important moments - breeding, hatching, feeding and hiding - by the light and rhythm of the moon.
Some of these events are among the most spectacular sights in the natural world. They also show how precisely life on Earth has learned to read the lunar calendar.
The Tides: A Quick Refresher
The moon's gravity pulls the ocean into two bulges, one facing the moon and one on the opposite side of the Earth. As the Earth turns, most coasts pass through both bulges each day, giving two high tides and two low tides. Because the moon moves along its orbit, the tides arrive about 50 minutes later each day - the same delay explored in moonrise and moonset.
Around the new moon and the full moon, the sun and moon line up and their pulls combine, producing spring tides with the greatest range between high and low water. Around the quarter moons, they pull at right angles, producing gentler neap tides. Many of the animals below use this fortnightly rhythm as their clock.
The Night the Reef Spawns
Once a year, on some of the world's great coral reefs, one of nature's most extraordinary events takes place. Over just a few nights, many coral species release their eggs and sperm into the water at almost the same moment. The sea fills with drifting pink, white and orange bundles, like an upside-down snowstorm rising toward the surface.
On the Great Barrier Reef, inshore reefs usually begin spawning between one and six nights after the first full moon of the southern spring, with outer reefs often spawning after the following full moon. The release happens only at night, typically a few hours after sunset.
Timing matters enormously. Coral cannot move to find a partner, so the only way for eggs and sperm from different colonies to meet is for everyone to release them together. Water temperature and day length bring the corals to readiness through the season. The moon then appears to act as the final signal, narrowing the event down to a handful of nights. Scientists are still unravelling exactly how corals sense the moonlight, but light-sensitive proteins in coral tissue are thought to play a part.
The Palolo Worm Rising
In the shallow waters around many Pacific islands lives the palolo worm, which spends most of its life hidden in coral rubble. Once a year, it performs a remarkable feat. The rear section of its body, packed with eggs or sperm, breaks away and swims to the surface, where it releases its contents into the sea.
The swarm happens at night during a waning moon in the southern spring, usually around the last quarter about a week after the full moon of October or November. Island communities have long tracked the moon to predict the rising and the arrival of the worms is celebrated as a seasonal delicacy and a festive occasion. It is one of the clearest examples of traditional lunar knowledge matching biological reality.
The Great Red Crab Migration
On Christmas Island in the Indian Ocean, tens of millions of red land crabs live in the rainforest. With the start of the wet season, they set off on a mass migration to the sea, crossing roads, rocks and gardens in a moving red carpet.
Their final step is timed by the moon. Females release their eggs into the ocean in the dark hours just before dawn, on the high tides around the last quarter moon, when the difference between high and low tide is smallest. Releasing at that point in the tidal cycle helps the eggs, which hatch the moment they touch seawater, drift safely away from the rocks. Weeks later, tiny young crabs emerge from the sea and march back into the forest.
Grunion: The Fish That Spawn on the Beach
Along sandy beaches of the eastern Pacific coast, small silver fish called grunion do something almost no other fish does: they leave the water to spawn. On certain nights, they ride the waves onto the beach and the females dig into the wet sand to lay their eggs while the males curl around them.
These grunion runs take place on the nights following a full or new moon, just after the highest spring tides of that cycle. The eggs then rest safely in the sand above the reach of the smaller tides that follow. Around two weeks later, the next set of spring tides washes over the beach, stirring the eggs and the young fish hatch and are swept out to sea. The whole life cycle is synchronised to the moon's fortnightly tidal rhythm.
Midnight Migrations in the Arctic
Every day across the world's oceans, a vast migration takes place. Countless tiny animals called zooplankton rise toward the surface at night to feed, then sink back into the depths at dawn to hide from predators that hunt by sight. It is often described as the largest daily migration of animals on the planet and it is normally driven by the sun.
But what happens in the Arctic winter, when the sun does not rise for weeks? In 2016, marine scientists published research analysing years of acoustic data from instruments moored across the Arctic. They found that during the polar night, the zooplankton switch to the moon. Instead of following a 24-hour day, they moved up and down on a cycle of about 24.8 hours - the length of a lunar day, from one moonrise to the next.
Around each full moon, when the moonlight was brightest, the migrating animals sank deeper, to around 50 metres, staying out of the light. The moon had taken over from the sun as the clock of an entire ecosystem.
Glowing Waves and Dark Nights
In many coastal waters, tiny organisms called dinoflagellates produce light when the water is disturbed. On the right night, breaking waves glow electric blue and every footstep along the shoreline sparks with light.
The moon plays a quieter role here. The glow itself is triggered by movement in the water rather than by the lunar cycle. But it is the darkness of the sky that decides how visible it is. Bioluminescent waves are most striking on moonless nights, around the new moon, when there is no competing light. Many people who travel to see glowing bays and beaches plan their visits around the new moon for exactly this reason.
Moonlight Guiding the Way
Moonlight also guides animals as they move. Newly hatched sea turtles, emerging from nests on the beach at night, head toward the brightest horizon - naturally the open sea, where moonlight and starlight reflect off the water. This instinct is why artificial lights near nesting beaches can disorient hatchlings and why many coastal communities dim their lights during the hatching season.
A Living Lunar Calendar
From reefs to beaches to the polar seas, the moon acts as a shared calendar for ocean life. It allows animals that cannot see or communicate with one another to meet at the same moment, to time their young for the safest tides and to hide from danger in the bright nights.
Humans have tracked the moon for festivals, farming and fishing for thousands of years, as explored in the three great lunar calendars. The ocean has been doing the same for far longer. Anglers still use the moon to time their trips, as described in fishing by the moon. The creatures beneath the waves have simply been following that rhythm all along.
By ML Chua