1. Many corals release eggs and sperm on one night, so that they can find each other
Most reef-building corals are "broadcast spawners": they let eggs and sperm go into the open water, where they have to meet. On the Great Barrier Reef off Australia, more than 130 coral species spawn on the same night, in a window of roughly 30 to 60 minutes[1]. Corals stay fixed in one place, so a colony cannot go looking for a partner. If every colony picked its own day, eggs and sperm would drift apart in the huge ocean. Releasing together is thought to raise the chances of fertilization[1].
The authors of one study call this one of the best examples of synchronized behavior on Earth[1]. It also shows up elsewhere. A Japanese-Taiwanese research team notes that in many places corals spawn around the full moon of early summer[2].
Season is set mostly by the sea getting warmer, which the same study names as the likely driver for ripening eggs and sperm[1]. But warmth alone cannot pick one night out of many. Something else has to point to the date.
2. Corals seem to read the light at night, and one study found the signal is a gap of darkness, not the Moon itself
The obvious suspect is moonlight, and there is evidence for it. In 2015, a team studied the coral Acropora millepora from the Great Barrier Reef. They gave corals different light treatments and sampled them before, during, and after spawning. Light changed gene activity linked to releasing eggs and sperm, and those changes appeared only on the night of spawning. When the corals were given light meant to mimic city light pollution, some of their cell signaling went out of step and they did not spawn. The authors conclude that moonlight is an important outside signal for synchrony[1].
A different experiment suggests a twist. In 2021, researchers working with the coral Dipsastraea speciosa at Green Island, Taiwan, covered corals at night to block moonlight. Uncovered corals spawned six nights after the full moon. Corals covered from three days before, one day before, or one day after the full moon each spawned five nights after the covers went on. So taking away night light moved spawning earlier, which means moonlight was holding spawning back, not calling for it[2].
Which light mattered? Tank tests with lamps showed that light in the first half of the night was what held spawning back. And when there was a dark stretch between the end of daylight and the start of night light, the holding-back effect disappeared[2].
Here is where the 50 minutes from the tides comes back. The Moon rises about 50 minutes later each day on average (the exact delay varies with the place and the date), so before the full moon it rises before sunset and its light follows straight on from the sunlight. After the full moon it rises after sunset, and a dark gap opens up in the evening. The researchers propose that this gap, which grows as moonrise slides later, is the corals' cue[2]. You can read the same 50-minute shift in why high tide comes about 50 minutes later each day.
The two studies look different on the surface: one stresses moonlight as a trigger, the other says moonlight is a brake and darkness is the go signal. They tested different corals under different conditions, so they may not be in direct conflict. Both agree that light at night matters and that artificial light can disturb it. Other things matter too. Temperature, tides, and the times of sunrise and sunset are also named as influences[1].
So a coral can read an outside cue. But many animals keep their dates even when the sky is cloudy or when they live where the Moon cannot be seen. Do they carry a calendar inside?
3. A tiny sea midge keeps a calendar inside its body, and it seems to work by counting days
Many sea animals time their lives by the tides, and the biggest tides come around full moon and new moon. Researchers have found body clocks that follow moon-related cycles, called circalunar (about a month) or circasemilunar (about two weeks) clocks, in algae, jellyfish relatives, worms, mollusks, crustaceans, insects, and fish[3]. The inner workings are mostly unknown. In more than 20 species with such a clock, researchers have tested how it works in only four[3].
One of those is a marine midge, Clunio marinus, a small insect of rocky shores. Its adults come out on a roughly two-week schedule. Three ideas had been proposed for how such a clock might work: a built-in 15-day oscillator, a counter that tallies days, or a "beat" produced when a daily rhythm and a tidal rhythm overlap[3].
A 2024 study put the midge in the lab with no Moon at all and changed the length of its "day". When researchers stretched the day-night cycle, the two-week rhythm stretched with it in a straight line. That fits a day-counting clock. When they used a 6-hours-light, 6-hours-dark cycle, the rhythm was about the same as with a normal 12:12 cycle, which suggests it follows the animal's own daily clock and not the outside light cycle. And when constant light threw the daily clock out of order, the two-week rhythm vanished too[3].
That goes against the long-held view that this midge's two-week clock runs independently of its daily clock[3]. Still, it is one insect. The authors suggest that the moon-related clocks of flies and mosquitoes may work on different principles than those of marine worms and algae, and that such clocks may have evolved more than once[3].
A clock that counts days could keep a two-week beat. But the sea itself rises and falls twice a day. Is there a clock for that?
4. Some shore animals keep a tide clock of about 12.4 hours that runs even without any tide
High tides come about 12.4 hours apart. Scientists call a body rhythm of this length a circatidal rhythm[4]. Some shore animals keep following it in a tank with no tides.
The best-studied case is a tiny crustacean, Eurydice pulchra, from European beaches. A 2013 study found that its swimming rose and fell on a cycle of about 12.4 hours in constant conditions, could be reset by suitable stimuli, and did not speed up or slow down with temperature. Those are the three tests used to decide whether something is a true biological clock. When the researchers disrupted the genes of the animal's daily clock, its tidal timing was not affected. They concluded that it has a dedicated tide pacemaker, separate from its daily clock[4].
Another example comes from Japan. A cricket that lives on the floor of mangrove forests, Apteronemobius asahinai, stays active on a 12.6-hour cycle when conditions are held constant. It also has a daily rhythm, being more active at subjective night. Researchers found 284 genes whose activity rose and fell on a cycle of 12 to 15 hours, and they note that their sampling was small and the results are a first step[5].
Whether a tide clock and a daily clock are really separate machines is still debated. Different animals may have solved the problem in different ways[4][5]. So the reef's answer has three layers: corals read outside light cues, a midge counts days inside its body, and some shore animals tick along with the tides. None of these is a brain, and each one does a job that a calendar would do for us.
How to look for these rhythms yourself
- Track moonrise. Find a free moonrise table for your town and write down the time of moonrise for ten nights in a row. Does it slip by about 50 minutes a day? On which night does moonrise move from before sunset to after sunset?
- Match it to the tides. Put a tide table beside your moonrise notes. Both slide by a similar amount each day. Which one changes more evenly?
- Notice night light. Look at how bright the sky is near your home after dark. Researchers worry that light pollution may confuse corals, and they suggest that reducing artificial light near reefs could help[1]. What light could be turned off near you?
- Visit a shore carefully. If you go to a beach or tide pool, go with an adult, check the tide times first, and look without picking up or moving the animals. Small shore animals may be running a clock you cannot see.
- Read the original. The eLife paper on coral spawning has a short plain-language summary at the top. Read it and compare it with this article.