Why Do Corals Spawn Around the Full Moon?
– A Quiet, Synchronized Event Under the Sea
On coral reefs, there are a few nights a year when countless corals release eggs and sperm into the water all at once, in a dreamlike spectacle. It looks like snow drifting through the sea, and it often happens on a set night a few days after the full moon. Corals cannot even move. So how do they manage to time it so precisely?
Corals live fixed to the seabed like rocks. Although they are animals, they cannot move around to send a message to their neighbours.
Yet on some reefs, researchers have observed corals of one species, or of many species, releasing eggs and sperm into the sea all at once, within just a few nights of the year. And those nights are often concentrated a few days after the full moon.
Corals cannot talk to each other. So how do they line up their spawning so precisely?
They are thought to sense the change in moonlight, which grows stronger toward the full moon, and use it as a cue for when to spawn.
By releasing huge amounts of eggs and sperm at the same time, they are thought to raise the odds of fertilization and improve their breeding success.
Let's look at how this "moon-timed group behaviour" works, step by step.
What Is Coral Mass Spawning?
Most corals are thought to be hermaphrodites, making eggs and sperm at the same time. When they spawn, they release small packets holding both into the water, and fertilization happens near the surface. In certain waters, a large-scale event has been observed in which not just one species but many species of coral spawn on the same night. It is called "mass spawning."
How Do Corals Sense Changes in Moonlight?
One cue corals use to synchronize is thought to be the change in the strength of night light as the moon waxes and wanes. Night light grows stronger as the moon nears full and weaker as it nears new. Corals are thought to have light-sensing machinery in their bodies, and to use this cycle of light as a "calendar". In practice, though, several environmental factors together set the timing, including seasonal changes in water temperature and day length.
They simply sense the same moonlight in the same way, and so act as if they had agreed in advance.
Why Do They Need to Spawn All at Once?
Coral eggs and sperm are simply released into the seawater, so there is a high chance that they never get fertilized. If each coral (or colony) spawned on its own schedule, eggs and sperm spread through the sea would have a very low chance of meeting. So when members of the same species all spawn on the same night, at the same time of night, huge amounts of eggs and sperm gather in a small area, and this is thought to greatly raise the chance of fertilization.
Interestingly, in many regions mass spawning is thought to occur not on the night of the full moon itself, but on a set night a few days later. It has also been reported to peak at a set time some hours after sunset, which suggests that several cues, such as darkness after sunset and water temperature, work together with the light cycle.
What You Can Check Yourself
- Use a calendar or a moon-phase app to check how many days it is since the new moon (the moon's age) today
- If you can, look up at the sky at the same time each night and record how the shape of the moon changes
- Work out roughly when the next full moon will be, using the cycle of the moon's phases (about 29.5 days)
- Check how close your guess came to the actual date of the full moon
Feel for yourself, through your own eyes and notes, the cycle of the moon's phases that corals are thought to use.
Summary
Coral mass spawning is thought to be timed by using the cyclical change in moonlight as an internal "calendar". When corals of the same species release eggs and sperm together, on the same night and at the same time of night, they raise the chance of fertilization and improve their breeding success. The exact timing of spawning is thought to involve not only moonlight but several environmental factors, such as water temperature and darkness after sunset.
Corals cannot move, but by looking up at the same moon, they line up an invisible moment with real precision.
For Those Who Want to Know More – Terms, Numbers and Links to TextbooksFrom middle-school science to active research, each topic is labelled by level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school Earth Science (basic)
- High school+High-school Biology, or advanced or sidebar material in textbooks
- UniversityNot taught in high school; university-level marine biology
- ResearchNot yet settled fact, even at university; questions researchers are studying now
Middle schoolTerms: Words Around Coral Mass Spawning
- Mass spawning: An event in which many corals spawn together on the same night.
- Moon age: The state of the moon's phase, given as the number of days since the new moon.
- Hermaphrodite: An individual that makes both eggs and sperm.
High schoolChecking with a Formula: Days Until the Next Full Moon
Using the cycle of the moon's phases (the synodic month) and the current moon age, let's calculate the days until the next full moon.
Days to next full moon = length of synodic month (days) − current moon age (days)
| Length of synodic month | The average number of days from one new moon to the next. It is taken to be about 29.5 days |
| Current moon age | The number of days counted from the most recent new moon |
| Days to next full moon | 29.5 − 12 ＝ 17.5 |
| Result | The next full moon comes in about 17 to 18 days |
| Days to next full moon | 29.5 − 20 ＝ 9.5 |
| Converted to weeks | 9.5 ÷ 7 ≒ 1.4 |
| Result | The next full moon comes in about 9 to 10 days, a little over a week |
The full moon falls at about half the moon's cycle (about 14.75 days), so the closer the moon's age is to this value, the closer it is to full. Corals are thought to use the change in light around the full moon as one cue for when to spawn.
* The synodic month is an average value, and in practice it is said to vary slightly.
High school+Why Spawning Together Pays Off: The Ecology
In ecology, it is thought that for species that scatter eggs and sperm over a wide area (broadcast spawning), the density of nearby individuals strongly affects fertilization success. Making the concentration of eggs and sperm extremely high in a short time through mass spawning is considered one effective strategy for raising that success.
UniversityHow Corals Sense Light: The Machinery Inside
In marine biology, researchers are investigating whether corals have light-sensing proteins (photoreceptors) that detect the strength of moonlight and small changes in it. It is thought that these photoreceptors may play a part in the body clock and in adjusting the timing of reproduction.
ResearchWhat Is Still Unclear
- Which photoreceptors corals use, and by what molecular mechanism they sense changes in moonlight, is a topic still under study in marine biology.
- The possibility that rising sea temperatures and other effects of climate change could shift or disrupt the timing of mass spawning is also considered an important research question for reef conservation.
- Research is also under way on techniques for restoring and growing coral reefs that draw on how mass spawning works.
The quiet mass spawning under the sea is full of rich themes, where astronomy meets ecology, and research is still going on.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: phases of the moon | Basic terms: mass spawning, moon age, hermaphrodite |
| High school | Earth Science (basic): motion of the moon | Calculating days to the next full moon from the synodic month |
| High school+ | Biology: population ecology (advanced) | Broadcast spawning and fertilization success |
| University | Marine biology | Sensing moonlight with photoreceptors |
| Research | Marine biology and conservation ecology (ongoing) | Molecular mechanisms, effects of climate change, conservation techniques |
- Explanations of coral mass spawning in marine biology textbooks and reference materials.
- Research reviews on broadcast spawning and fertilization success in the field of ecology.
- Research reviews on coral photoreceptors and body clocks in the field of marine biology.
- Explanations of the cycle of the moon's phases (the synodic month) in Earth science textbooks.
- Explanations of climate change and its effects on coral reefs in conservation ecology materials.
* The timing and conditions of spawning are said to vary with coral species, region and the environmental conditions of the year.
* This article is a general science explainer. When observing coral reefs, take care not to touch or damage the corals or the creatures around them.