I dove off the back of our sailboat into the warm waters of the South Pacific just after sunrise. My husband and 10 year-old son followed, each holding an underwater camera. Together, we kicked over to a nearby coral reef.
This wasn’t our first time snorkeling off the gorgeous island of Taha’a in French Polynesia. My family had already spent two weeks exploring the Society Islands abroad a catamaran that we’d chartered from Dream Yacht Worldwide.
But it was our first time snorkeling as citizen scientists. Our task: document whether the coral below sent up a cloud of eggs and sperm. This is called broadcast spawning, and it’s how corals have sex to reproduce and create reefs.
We spread out, hovering atop mounds of coral teeming with colorful fish. Then we waited to see if the reef released its stuff.
We weren’t the only ones snorkeling that morning in search of eggs and sperm. On January 18, more than 400 observers from 20 countries volunteered to watch coral spawn. Snorkelers were spread across 11,000 miles of the Pacific, spanning both hemispheres, from Polynesia to Tanzania. Our combined observations are helping to track the health of coral reefs across half the world.
“Coral spawning is the only way for a reef to be resilient. You get a new generation of corals, and this new generation can adapt to its new environment,” says Vetea Liao, a marine biologist based in French Polynesia who organized the first-ever worldwide observation event called Connected By The Reef.
This global citizen science effort focused on an important reef-building species of coral called Porites rus. This is the only species of coral known to reproduce in the daylight instead of at night.
It also spawns like clockwork: five days after the full moon, 90 minutes after sunrise during the warm season.
Liao discovered this phenomenon in November 2014 while spearfishing near his home in Tahiti. “I was really surprised. I didn’t know coral could spawn during the day,” he says.
The Only Coral Species Known To Reproduce In Daylight
No one else did, either. Liao couldn’t find any scientific studies about coral spawning in sunlight instead of at night. So, he organized friends around French Polynesia to help him watch the reef to confirm the phenomenon. In November 2019, they documented Porites rus spawning around multiple islands at the same time.
But since most coral species only spawn once each year, Liao assumed Porites rus was also an annual spawner. Then a friend told him that “the water was very cloudy” in December, too. Liao discovered that this type of coral spawns much more frequently than other species.
In 2021, Liao founded the nonprofit association Tama no te Tairoto (Children of the Lagoon) to engage more people in collecting data on coral spawning. The global observation event in January 2025 required plenty of planning and coordination, including media outreach, developing a new app for volunteers to record their data, and creating an international network of snorkelers who knew what to look for.
My family learned about the event from an article in Air Tahiti’s in-flight magazine. As avid snorkelers and world-schoolers of two children, my husband and I bookmarked the date and time in our calendars. What better way to foster future stewards of the ocean than engaging kids in collecting useful information about its inhabitants?
At 07:32, I saw milky white puffs emerging from the reef below me. Excited, I waved over my son and husband. We took turns diving down to snap photos, moving between reefs as they pumped out millions of larvae that will spread around the ocean to create new reefs that support all sorts of underwater critters.
“There’s a genetic superhighway of coral information that’s flowing across the oceans that we have no idea about until we actually go and study these things,” says Peter Molnar, co-founder of the Ocean Genomic Atlas Project that aims to identify, sequence and catalog the millions of tiny plankton swimming in our seas, including coral larvae.
We swam back to our sailboat to record our observations and upload photos in the mobile app, happy that our hobby helped further efforts to understand the seas.
What Triggers Synchronous Coral Sex?
A few weeks later, Liao emailed the volunteers an initial summary of the findings. Spawning was confirmed in more than a dozen countries. For the first time ever, observers documented Porites rus spawning simultaneously in both the Southern Hemisphere and the Northern Hemisphere. Another fascinating discovery from the worldwide observation event was that spawning was delayed by four hours for Porites rus colonies observed at depths of 40m and below.
In some countries, observers did not document spawning, such as in the Cook Islands, Guam, and the Philippines. Other regions, like the Maldives and Tanzania, had a mix of results, with some sites spawning and others not. This could mean that not all coral colonies reproduce in the same month, or that local environmental factors influence spawning, such as the presence of freshwater after a recent rain.
The big question to answer next: what triggers this synchronous coral sex? Hypotheses range from wind or atmospheric pressure to water chemistry or the timing of sunlight. Once we know what gives the green light for the coral to reproduce, we can better target ways to protect this important marine resource.
“It’s really a big scientific mystery,” Liao says, which volunteer snorkelers and divers are helping to unravel around the world. “Citizen science is very powerful. You can have many eyes in the water at the same time. It’s a really fast way to get concrete actions towards protecting coral reefs.”
