Before the Moon Comes Up

A coral colony cannot move to find a mate. On a few nights a year it releases its eggs and sperm into the sea, and if its neighbours do the same within the same hour, some of them meet. The night, out of all the nights, is set by the Moon. How a coral counts nights is still being worked out, and one recent answer is testable against every spawning anyone has written down.

1. Three clocks

In 1984 six biologists working on the Great Barrier Reef (Peter Harrison, Russell Babcock, Gordon Bull, James Oliver, Carden Wallace and Bette Willis) published "Mass spawning in tropical reef corals" in Science: many species there release their gametes together, on the same few nights. Two years later, with largely the same authors, came "Synchronous spawnings of 105 scleractinian coral species on the Great Barrier Reef". The pattern turned out to run on three clocks: a season, roughly the month; a night, counted from the full moon; and an hour, counted from sunset. Divers have been recording it ever since, and in 2021 Andrew Baird and 90 co-authors gathered those records into the Coral Spawning Database: 6,178 observations from 181 sites across the Indo-Pacific, released into the public domain (CC0).

Each record gives a place, a date and usually a start time. That is enough to put the sky back over it. Everything on this page recomputes the evening each record happened on, sunset and moonrise at that reef on that date, with an ephemeris checked against NASA JPL's (section 5), running in your browser.

One reef, one lunar cycle, every evening (n = records that evening)
sun upMoon above the horizondarka recorded spawning start

Rows are evenings, counted from the full moon's local calendar date (the database's own convention). The pale band in each row is the Moon: before full moon it is already up at sunset; after full moon it rises later every evening, by a median of 45.9 minutes at these sites (10th to 90th percentile 34.3 to 58.2), and a dark gap opens after dusk. Records without a start time are counted in the row label but not drawn.

2. Every timed spawning, with its Moon

Put all of them on one picture. Across the axis, nights after the full-moon date; up the side, hours after that evening's sunset. The line is the median moment the Moon rose on those evenings at the reefs concerned, and below it the sky was dark.

Start times against the dark gap
began with the Moon below the horizonbegan with the Moon upmedian moonrise after sunset

Of the 4,189 spawnings with a start time in the evening, 3,302 (78.8%) began with the Moon below the horizon. On the busiest nights it is nearly all of them: 628 of 679 on the fourth night, 776 of 788 on the fifth, 629 of 632 on the sixth. This is a description, not yet a test. Spawning a few hours after sunset, several nights after full moon, would land in the dark whatever set the night, because by then the Moon rises late. The question is what the coral is counting.

3. A timer set by the first dark dusk

In 2021 Che-Hung Lin, Shunichi Takahashi, Aziz J. Mulla and Yoko Nozawa, from Academia Sinica in Taipei and the University of the Ryukyus, shaded colonies of the coral Dipsastraea speciosa in the field on different nights around the full moon. Their abstract:

"When corals in the field were shaded 1 and 3 d before the full moon or 1 d after the full moon, spawning always occurred 5 d after shading commenced. [...] In nature, moonrise gets progressively later during the course of the lunar cycle, shifting to after sunset following the day of the full moon. Our results indicate that this period of darkness between sunset and moonrise triggers synchronized mass spawning of D. speciosa in nature."

They wrote that they expected the model to apply to other corals. In 2025 R. de la Torre Cerro and thirteen colleagues darkened the post-sunset sky over the table coral Acropora aff. hyacinthus in Palau, and found that corals kept dark after sunset "for at least two to three consecutive nights advanced their spawning compared to controls". Both are experiments, which the database is not. But the database can ask a question neither experiment could: in the wild, across hundreds of reefs and forty years, does the spawning night move when the first dark dusk moves?

The first dark dusk does move. It usually comes on the evening of the full-moon date or the one after, and which one depends on when in the day the Moon is full and on where the Moon is in its tilted orbit relative to that reef's latitude. So nature runs the experiment: in some cycles the first dark dusk falls a day later than in others, relative to the same calendar date. Here is how often, for each hour of the day the full moon can fall on, across the database's 727 lunar cycles (a reef and a full moon).

When is the first dark dusk a day late?
first dark dusk on the full-moon datethe day after

Each bar is one hour of the day at which the Moon was exactly full, local time, and the lunar cycles at the database's reefs whose full moon fell in it. When the Moon was full before 10:00, the first dark dusk came that same evening in 299 of 300 cycles (the exception, at Kochi, 33°N, came the evening before). From 21:00 on it came the next evening in 82 of 84. In between, geometry decides.

If the timer is real and general, a species on a reef should spawn one night later, counted from the full-moon date, in the cycles where its first dark dusk came one day later. The test is a single slope, called β here: the shift in the spawning night when the first dark dusk slips by a day, measured within each species on each reef so that no species is compared with another. β = 1 is the timer. β = 0 is a spawning night that ignores the dark dusk. The uncertainty is a bootstrap over spawning events rather than records, because a single mass spawning can be dozens of rows.

The step the timer predicts

Each point is the average, over spawning events whose full moon fell in that three-hour window, of how many nights later than its species-on-this-reef average the spawning came. Bars are ±1.96 standard errors across events. The timer predicts a rise of about one night across the afternoon and evening, where the first dark dusk slips to the next day. A flat line is a spawning night that did not notice.

4. What the database said, in the order it was asked

The order matters, so here it is. I ran the test on all the records first, then broke it down by region and looked at six regions one at a time. One of them, the central and northern Great Barrier Reef, the best-sampled region in the database, showed the step clearly. A result found by looking at six subgroups is the kind of thing chance produces, so before running anything else I wrote down a test on the regions I had not looked at, with the code, the decision rules and my expectation, and pushed it to this project's history (PREREGISTRATION.md, 2026-09-26 06:38 UTC; the history itself is private, the file is here). Only then did I run it.

Every estimate of the slope, with its 95% interval

The pre-registered answer. On the regions held out, β = −0.40, 95% interval −1.24 to 0.41, over 217 spawning events. The interval excludes 1. Across those reefs, when the first dark dusk came a day late, the spawning night did not.

It is what I expected, and wrote down that I expected, because those regions were already inside the pooled figure: all records together give β = 0.07 (−0.37 to 0.48, 454 events), an interval narrow enough to tell 0 from 1.

The Great Barrier Reef result does not survive a harder look. Its β of 0.73 has a permutation p-value of 0.028 (shuffling which spawning events had a late dark dusk, within each region, 1,000 times). That was one of six regions examined, and six looks at a p of 0.028 is roughly one chance in six of seeing it somewhere by luck. Demanding at least 30 minutes of darkness after sunset before calling a dusk dark, which is closer to what a coral could plausibly sense, drops it to 0.36 (−0.09 to 0.76). The nearest thing to a replication, the southern Great Barrier Reef, pre-registered as a secondary test, points the other way on 22 events. The one secondary that agrees is Palau, where the 2025 darkening experiment was done: β = 0.84, on 11 events, with an interval running to 4.

Neither can the database settle the species the timer was found in. It holds 67 records of Dipsastraea speciosa, 18 of them from Lyudao (Green Island), Taiwan, contributed from Lin and Nozawa's 2017 study, and in every Lyudao cycle recorded the first dark dusk fell on the full-moon date. With no cycle that slipped there is nothing to compare, and across all its sites the species gives an interval from −3 to 3.

And the other obvious clock does not win either. A coral that tracked the Moon's phase continuously would spawn later, counted from the calendar date, the later in the day the Moon was full. Fitting that and the dark dusk together (an exploratory analysis, run after the pre-registered ones), the dark dusk's coefficient is −0.17 (−0.73 to 0.38) and the phase coefficient 0.47 (−0.19 to 1.05). The two are too entangled in these records to separate, and in no version of the analysis does the dark dusk carry the one-night shift the timer predicts.

5. The database's own arithmetic, checked

The database carries its own night count for every record, built from a table of full-moon dates it ships alongside. Both can be checked against the sky.

6. What this cannot tell you

The check