Why Do Corals Mass Spawn Simultaneously? The Key Lies in the Dark Window Between Sunset and Moonrise!

The Editor says: Last year, the Editor witnessed coral spawning in Kenting for the very first time and was utterly awestruck by the breathtaking spectacle and the sheer vitality of the corals! What mechanism drives corals to mass spawn all at the same time? Let's follow the Academia Sinica "Research Digest" interview to find out! <Full article republished from Academia Sinica Research Digest – Why Do Corals Mass Spawn Simultaneously? The Key Lies in the Dark Window Between Sunset and Moonrise! >

Unravelling a 40-Year Mystery of Synchronized Coral Spawning

As far back as the 1980s, scientists discovered that corals have a remarkable sense of coordination — they reproduce together within a short window of time, flooding the surrounding waters with vast quantities of coral eggs and creating a breathtaking spectacle. Researchers had long speculated that this synchronized spawning was influenced by factors such as temperature, tides, and light, yet the precise trigger had never been confirmed. After 40 years of effort, a team at the Academia Sinica Biodiversity Research Center has finally cracked the secret! Academia Sinica's "Research Digest" sat down with Associate Research Fellow Yoko Nozawa and Postdoctoral Researcher Lin Che-hung, who discovered that the key to synchronized coral spawning lies in the dark period between sunset and moonrise.

Surprising Facts: Corals Are Actually a Group of Animals with Remarkably Diverse Forms!

Because corals can only attach themselves to fixed locations and cannot move, they were once mistakenly classified as plants. Their appearance also tends to mislead people into thinking that one large coral colony is a single organism. In reality, however, most corals are colonies of individual polyps; only a few species — some within the family Fungiidae, for example — consist of a single giant polyp forming one individual coral.

Take reef-building corals as an example. A coral polyp colony can be divided into two parts: the non-living and the living. The coral skeleton, composed of calcium carbonate, serves as a protective shell and home; covering it are countless living, active coral polyps.

Coral polyps are classified in the phylum Cnidaria. In appearance they resemble sea anemones — fellow members of the same phylum — featuring a cylindrical body with a single opening surrounded by tentacles densely packed with nematocysts, which they use to capture plankton for food. Another food source for coral polyps is provided by their mutualistic symbiotic zooxanthellae, which carry out photosynthesis to produce nutrients and oxygen, while also lending the white coral skeleton and otherwise transparent polyps their vivid colours.

Every coral we see in the ocean — large or small — originates from a single tiny polyp that divides and divides again. Coral polyps engage in asexual reproduction continuously, splitting into vast numbers of individuals over the years. The calcium carbonate secreted by countless generations of polyps accumulates layer upon layer until it forms a castle-like structure: the coral reef. Scientists regard coral reefs as the tropical rainforests of the sea, providing habitat and abundant food and energy for fish, crustaceans, and a host of other organisms.

Associate Research Fellow Yoko Nozawa considers coral to be a truly extraordinary life form. Starting from a single polyp so small it is invisible to the naked eye, it divides and reproduces without pause until billions of polyps come together to form the only living structure on Earth visible from outer space: the Great Barrier Reef.

However, the new individuals produced through division are genetically and morphologically identical to the previous generation. This kind of asexual reproduction cannot increase genetic diversity and eventually leaves populations unable to adapt to environmental change. Corals must therefore expend time and energy releasing sperm and eggs through sexual reproduction to create offspring with new genetic combinations.

Do Corals Understand Risk Management? The Spawning Strategies of Spreading Bets vs. Going All-In

Unlike fish, which can find a mate before releasing eggs for fertilisation, sessile corals have no choice but to release their gametes directly into the water. To overcome the disadvantage of immobility, they adopt a synchronized strategy — agreeing, so to speak, to release staggering quantities of sperm and eggs together within a short time frame. This dramatically increases gamete concentration and therefore the chances of successful fertilisation; even if predators lurk nearby hoping for an easy feast, they are quickly overwhelmed and can't keep up.

The stunning spectacle that humans see during coral spawning is simultaneously a testament to life's effort to overcome the challenges of nature in order to reproduce.

Synchronized coral spawning can be further divided into two patterns. Yoko Nozawa points out that corals spawn only once a year. Some species prefer to spread their risk: colonies within a species spawn simultaneously, but different colonies stagger their timing by a few days before or after. Other corals, by contrast, go all-in — every colony agrees to spawn on exactly the same day. The latter approach naturally offers a higher probability of fertilisation, but if a typhoon, heavy rain, or other adverse weather strikes on that precise day, virtually no offspring may survive that year.

"The fact that both strategies have evolved tells us each has its own advantages," Yoko Nozawa explains. But regardless of whether a species is cautious or a risk-taker, how do corals — unable to move or communicate with one another — manage to coordinate spawning? Since the phenomenon of synchronized spawning was first described in 1980, this mystery has puzzled the world for a full 40 years.

Seven Years of Field Data Reveal: The Key Factor Is Hidden in the Lunar Cycle

Beginning in 2010, Yoko Nozawa's research team has traveled to Green Island every year during the coral breeding season (typically April, May, and June in southern Taiwan) to conduct underwater surveys. During each survey period, the team dives every night to record coral species, colony numbers, and spawning times. After accumulating seven years of data, Postdoctoral Researcher Lin Che-hung identified clear reproductive patterns for each coral species.

According to the team's records, corals in the family Merulinidae adopt a risk-spreading strategy, with different colonies spawning in synchronized batches. Although the timing is staggered between colonies, it follows a very consistent schedule: always five to eight days after the full moon. Green Island is also home to a large number of corals belonging to the genus Acropora, which all agree to spawn on the same day — yet the exact date varies from year to year.

"Merulinidae always spawns five to eight days after the full moon; Acropora also spawns after the full moon, but with no apparent pattern," says Lin Che-hung. Even so, both groups spawn after the full moon, so the research team focused on a lunar-cycle factor — moonlight — and set out to test its role.

Results from Both Lab and Field Consistently Show: Nocturnal Light Suppresses Coral Spawning

Because Dipsastraea speciosa (family Merulinidae) is common on Green Island — easy to observe and sample — and its reproductive timing follows a predictable pattern, the team chose this species for their experiments. "When moonlight was blocked out, Dipsastraea speciosa spawned earlier," says Yoko Nozawa. The preliminary results suggested that the darkness following the full moon is the environmental cue that tells corals to prepare for spawning.

To eliminate interference from other environmental factors, experiments were first carried out in aquarium tanks in the laboratory. The team then moved to the area near Gongguan on the northern tip of Green Island to confirm that corals — whether in an artificial setting or their natural habitat — would spawn earlier when exposed to darkness. "We dived every single day, covering corals with opaque aluminium foil sheets or transparent sheets three days before the full moon, one day before, and one day after," says Lin Che-hung. The results matched expectations: the earlier corals were covered with the blackout cloth, the sooner they spawned — reliably mass spawning five to eight days after receiving the darkness signal.

Light of Different Wavelengths Produces the Same Suppressive Effect

Beyond the simple presence or absence of light, Lin Che-hung also investigated the spectral composition and intensity of light sources. A 2006 paper published in the journal Science had suggested that corals may be able to detect moonlight. Yoko Nozawa noted that the paper described how exposure to moonlight causes corals to express the cry gene, and that the cry gene responds particularly strongly to blue light.

The team therefore returned to the laboratory and used artificial light sources to simulate moonlight intensity, exposing corals separately to red, blue, and green light to determine whether different wavelengths would produce different levels of stimulation, as the literature described. The experiments showed, however, that under all three colours of light, corals equally refrained from spawning. In other words, all the evidence gathered so far points to the same conclusion: darkness is the key to coral spawning.

The Answer to the 40-Year Coral Mystery: The Dark Window Between Sunset and Moonrise

After systematically working through the evidence, the team confirmed that nocturnal light suppresses coral spawning. But they wanted to go further — did even a momentary flash of light during the long night interfere with corals, or did suppression require prolonged exposure? To find out, the team conducted laboratory experiments examining four scenarios individually: complete darkness all night, light all night, light during the first half of the night (sunset to midnight), and light during the second half of the night (midnight to sunrise).

The results showed that the group exposed to light only in the second half of the night spawned synchronously after five days — just like the group kept in complete darkness all night. Light during the first half of the night, however, produced the same effect as all-night light exposure: delayed spawning and reduced synchrony. "Seeing this, we hypothesised that the coral's light-sensing receptors must have 'operating hours,'" says Lin Che-hung with a laugh. Those operating hours appear to fall between sunset and midnight, though there is some variation among individual coral colonies.

The answer was finally revealed: in the case of Dipsastraea speciosa, just two consecutive nights with approximately one hour of darkness after sunset are sufficient to trigger the conditions for synchronized spawning. This also explains why corals consistently choose to reproduce after the full moon. Lin Che-hung points out that because the Earth rotates while the moon orbits the Earth, the time of moonrise is delayed by roughly 30–70 minutes each day (Note 1). Mapping this against the lunar cycle in April during the breeding season, at the beginning of the month the moon rises around 2 p.m.; it then rises progressively later each day until, by the full moon, it rises only after sunset — and that dark window in between is the signal telling corals: it's time to spawn.

There are good reasons for spawning after the full moon, Yoko Nozawa points out. Dipsastraea speciosa spawns during darkness and neap tides: the dim conditions partially obscure the corals from predators, and the calmer seas of a neap tide mean gametes are less likely to be immediately dispersed.

After Receiving the "Dark" Signal, Coral Eggs Need Five Days to Ripen

As for the microscopic mechanisms behind Merulinidae's consistent five-to-eight-day post-full-moon spawning schedule, the research team is still actively investigating. It likely relates to the maturation processes of the sperm and eggs. The following is the team's hypothesis based on their observations.

Merulinidae are hermaphroditic: each polyp produces both sperm and immature eggs. When a coral receives the stimulus of two consecutive nights of darkness, the nucleus of the egg cell gradually migrates toward the edge of the oocyte — a process called germinal vesicle migration (GVM), which takes approximately five days.

Once GVM is complete, the egg cell nucleus begins to break down over roughly three to four hours in a process called germinal vesicle breakdown (GVBD), at which point the oocyte is nearly ready for fertilisation. The mature eggs and sperm are then packaged together into structures called "gamete bundles." Yoko Nozawa notes that after the coral releases these bundles into the water, they float up to the surface — because the probability of sperm meeting egg in the two-dimensional plane of the sea surface is far greater than in the three-dimensional space below.

The gamete bundles break apart at the surface, and the released eggs have only one final step remaining: to expel the polar body from within the cell before they can unite with sperm. Interestingly, freshly released eggs preferentially fuse with sperm from a different coral colony; but as time passes, they will accept sperm from the same colony. "Otherwise, if they wait too long, the gametes will either be dispersed or eaten, and the chance of fertilisation only grows slimmer," Lin Che-hung adds.

After successful fertilisation, the fertilised egg sinks into the water and develops into a ciliated, free-swimming planula larva. The planula spends several days searching the seafloor for a suitable spot; once it finds one, it settles, metamorphoses, and becomes a coral polyp that can no longer move freely. The polyp then divides continuously and secretes calcium carbonate, gradually growing into a coral colony.

A Fortuitous Encounter Brings Years of Research to an International Journal

"It was really a stroke of luck — the paper had originally been submitted to a different journal," says Lin Che-hung, the paper's first author, with a smile. A visit to Taiwan by Japanese scholar Takahashi Shunichi unexpectedly gave this new discovery about coral spawning the opportunity to be published in the Proceedings of the National Academy of Sciences (PNAS).

When Professor Takahashi Shunichi of the University of the Ryukyus was staying at Academia Sinica, he dropped by the lab of his fellow countryman Yoko Nozawa. In the course of their conversation, they discovered they had actually been university classmates. "We barely knew each other back then — just nodding acquaintances — and had had no contact since graduation," says Yoko Nozawa. Takahashi had gone on to conduct research in tropical biology, genetics, and molecular science at the University of the Ryukyus, while Nozawa had divided his time between Academia Sinica and Green Island, studying coral ecology and behaviour. Neither had expected to cross paths again in academic circles.

At Takahashi's suggestion, the two teams collaborated to expand and strengthen the experiments. Lin Che-hung recalls that Takahashi offered several tips on designing experiments and submitting to journals — such as replicating results in both laboratory and field settings to increase the persuasiveness of findings; writing conservatively when drafting the paper, sticking only to what has been firmly established rather than overreaching; and taking care with the wording and organisation of the manuscript while maintaining patience in communicating with reviewers.

This serendipitous connection led to a Taiwan–Japan cross-border collaboration and gave Yoko Nozawa, Lin Che-hung, and their colleagues the opportunity to see their years of diligent research published in a high-impact journal — bringing the truth about coral spawning to a much wider audience.

A Difficult Ecological Study Turns a Corner — An International Team Sets Out Again

Looking back on how his interest in scuba diving led him to choose coral as his research subject, Yoko Nozawa — more than 20 years on — has gradually come to hope that his work can contribute something to the ever-declining coral populations of the world. "I'm very happy to be doing research here. The support of Academia Sinica means I can focus without worrying," he says.

With the mystery of Dipsastraea speciosa's synchronized spawning now solved, Lin Che-hung is set to join the laboratory of Takahashi Shunichi — his current supervisor's old classmate — at the University of the Ryukyus, where he will begin a new coral research project. Yoko Nozawa says he will continue to support Lin Che-hung's postdoctoral research, since this study focused primarily on Dipsastraea speciosa; the team still wants to know whether other species within Merulinidae are also triggered to spawn synchronously by darkness, and whether there are further secrets behind Acropora's irregular post-full-moon spawning pattern and its unusual failure to spawn in the absence of light.

Also worth mentioning: following the publication of the coral spawning findings, Yoko Nozawa received a letter from Levy Oren, a researcher at Bar-Ilan University in Israel. Levy Oren studies the effects of light pollution on coral populations in the Red Sea and expressed great interest in the published research, along with hope for a future collaboration. Coral spawning research has long deterred many scientists — with only one annual observation window, nightly dive surveys, and high logistical risks, the work is extraordinarily demanding. Now, the years of perseverance by Yoko Nozawa, Lin Che-hung, and their colleagues have paid off — and a grand coral-rescue adventure spanning the Red Sea, Green Island, and the Ryukyus awaits them just ahead.

Note 1: Because the moon's orbit around the Earth is not a perfect circle, the daily delay in moonrise time varies depending on the lunar phase (new moon / full moon) and the season, ranging from approximately 30 to 70 minutes.

Full article republished from Academia Sinica Research Digest – Why Do Corals Mass Spawn Simultaneously? The Key Lies in the Dark Window Between Sunset and Moonrise!

Interviews and writing│Lin Cheng-hsun, Jian Ke-zhi
Art and design│Lin Hsun-an, Tsai Wan-chieh

Further reading:

• Lin, C.-H., Takahashi, S., Mulla, A. J. & Nozawa, Y. (2021). Moonrise timing is key for synchronized spawning in coral dipsastraea speciosa. PNAS, 118(34).
• "Coral Facts." NOAA Coral Reef Conservation Program.
• "How Do Corals Reproduce ?" Global Foundation for Ocean Exploration.
• "What are corals ?" National Ocean Service.
• 3 Things You Should Know About Photographing Coral Spawning