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By Rodrigo Santos Andrade
Spawning coral release quintillions of eggs each year: new research
visibility comment0 event October 13, 2026 timer 16:22
The first estimate of coral egg numbers across the Indo-Pacific shows which species can help rebuild devastated reefs.
Auscape / Getty ImagesMost kinds of coral spawn once a year. In an enormous multi-day event synchronised to specific phases of the moon, they release vast slicks of eggs and sperm into the ocean to be mixed together by the drifting currents.
Imagine hundreds of thousands of Olympic swimming pools filled with tiny pinkish spheres, all dumped into the sea at once.
In new research published in Ecology and Evolution, my colleague and I have for the first time estimated just how many eggs are involved in one of these spawnings, and it’s a mind-boggling number.
According to our calculations, in a single spawning event, corals across the Indo-Pacific region release around 3.5 quintillion eggs (that’s 3,500,000,000,000,000,000 if you write out all the zeroes).
This isn’t a just a curious piece of trivia, either: it’s basic information we need to help preserve Earth’s coral species for the future.
Coral spawn form large pinkish slicks in the ocean. Credit: Philippe Max Rouja.
Why count coral eggs?
Counting Earth’s ecological assets reveals how our planet functions. Knowing the total number of ants living in the ground (around 20 quadrillion) allows scientists to estimate how much carbon they can remove, for example.
Scientists have mapped half a trillion adult coral colonies, as well as tracking global fish egg production. These numbers tell us about the function and health of ecosystems.
Counting nature also tells us about its power to regenerate. Ocean life, and coral reefs in particular, are bearing the brunt of global warming. Since 1950, roughly 50% of global coral reef cover has disappeared.
This is why we want to know about coral’s capacity to reproduce and regenerate.
A baseline for coral reproduction
Our new study provides the first estimate of coral reproductive potential across the Indo-Pacific region, which is a hotspot for marine biodiversity.
Our estimate of 3.5 quintillion eggs released in a spawning establishes an essential baseline of what these reefs produce at peak capacity.
Measuring across hundreds of coral species, we found most eggs came from a few key groups. Branching Acropora, Porites (bommies or massive boulder corals), and Montipora accounted for around half the total egg supply.
How to count a quintillion eggs
How did we figure this out? We were looking at almost 190,000 square kilometres of tropical reef, so we couldn’t just swim out with a clipboard and start counting eggs.
Instead, we looked at what we knew about coral and eggs at the small, medium and large scales.
First we examined coral polyps. A single coral is a colony of many, many individual polyps. We calculated how many eggs fit inside the average polyp.
Next we estimated how many polyps are in each whole coral. This let us work out how many eggs an average-sized colony of each coral species produces.
Finally, we combined these figures with real-world data about coral cover and geographic distribution for hundreds of species.
This might seem like a straightforward process, but direct counts of eggs only exist for a handful of coral species. It’s a tricky task, and for many species we still don’t even know exactly when they spawn.
To fill in the gaps in the data, we made estimates based on patterns in data from related species.
Not all coral species are equal
We discovered many interesting things about coral egg production. Massive boulder corals produced the most eggs per square centimetre of living tissue, for example, while branching corals dominated overall egg production across the region.
So, our research uncovered an important scale mismatch. This means that polyp or colony-level output does not necessarily translate to high regional egg output.
We also found that while some groups of species produced fewer eggs on average, there were individual species within those groups that were highly productive egg factories.
Even within the group of branching Acropora species, one species produced ten times more eggs per square centimetre than the others.
This kind of species-level information tells us about what might be the true workhorses of reef repair. We need more of it – and that means genuine species-level measurements.
What now?
Why is this so critical for reef conservation? Reproduction is the fundamental bridge to recovery after severe marine heatwaves. Damaged reefs cannot recover without a supply of fresh coral babies onto the reef.
Our study provides a practical, data-driven blueprint for coral protection and rehabilitation. Knowing that a handful of coral groups carry most of the potential for repair, managers and conservationists can deliver their attention to these heavyweights.
Our work is what is called a “first-order” estimate: a rough draft with a large (but quantifiable) amount of uncertainty. It’s a good start, as it allows us to get an idea of the scale of coral reproduction and the differences between coral groups.
We now need to refine our estimates by going out and directly measuring egg release across new and diverse coral species. It’s an exciting but also daunting prospect – 190,000 square kilometres is a lot of area to cover with a clipboard.
Kate Quigley receives funding from the Australian Research Council and Minderoo Foundation. She is also Principal Research Scientist and Senior Advisor at Minderoo Founation.
Tags: Academic Journalism, The Conversation, The Conversation Academic News
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