Sex in the Sea
Timing is everything, especially for baby-making corals
Sure, you may keep a meticulous calendar, but even the perfectionists among us cannot rival the punctuality of spawning Acropora palmata. Elkhorn corals have an entry in their August calendar for 10:20 p.m. on the fourth night following a full moon, and last week to the minute, as practitioners working to rebuild reefs in Florida Keys National Marine Sanctuary watched in wonder, the corals answered their mating call.
“They’re somehow synchronized to the setting of the sun and the rising of the moon,” said Ken Nedimyer, Technical Director for Reef Renewal, USA. “Who knows? But it’s amazing.”
As one of the pioneers of coral restoration techniques, Nedimyer has seen nearly everything, but he was genuinely giddy over the 2025 spawning that took place under the cover of darkness in Reef Renewal’s Tavernier nursery, where the event had never been seen in the reef-building elkhorn species. Coral fragments that hang from tree-like structures are usually moved to the reefs or land-based locations before they reach sexual maturity, but a confluence of events this year left hundreds of basketball-sized elkhorn coral in Reef Renewal’s nursery as the calendar turned to spawning season.
“We sent the bigger corals to the University of Miami this year, and in years past we’ve sent some to the Florida Aquarium,” Nedimyer explained. “When they said they couldn’t take any more, we said, ‘Well, let’s go out and watch them and see if they spawn.’”
Sex in the Sea
On two consecutive nights at exactly 10:20, hundreds of thousands of gametes were released by the corals, floating into nets that Nedimyer’s team fitted like hoods over the large fragments.
“They’re called bundles,” he explains, “and each polyp pops out this bundle that consists of five to 10 eggs, and then in the middle is a packet of sperm. They’re hermaphrodites, so they produce both. Some people say the bundles look like Dippin’ Dots, or a little ball of Styrofoam. They float to the top, and they drift around for about half an hour to an hour before they break open, and then the sperm try to find an egg. They’re not really self-fertile, so they have to find the egg from a different genotype.”
The sperm and eggs are reproductively viable for only a few hours, so Reef Renewal rushed them to shore and then to the University of Miami, where the mixing and matching took place. A year from now, they may be large enough to return to the Keys as coral fragments, waiting to be part of NOAA’s Mission: Iconic Reef’s restoration program.
An evolution, revolution
“They were grouping selected gametes,” said Dr. Katey Lesneski, Mission Iconic Reefs Research and Monitoring Coordinator, “and then intentionally mixing them with gametes collected at Elbow reef, one of the few reefs where live colonies of reproductive-size elkhorn coral still exist. That’s a sure-fire way to create new genetic combinations that may not happen out on the reef naturally. A number of the genets in these nurseries are putatively heat resistant, so continuing those lineages and mixing them with new lineages is another way to ensure continuation of traits, and even potentially providing different beneficial traits.”
Nedimyer believes elkhorn is the only coral that can rebuild the reefs quickly enough to keep pace with sea level rise. While heat waves have decimated the species in recent years, the babies born in last week’s spawning are from parents who have demonstrated a resistance to thermal increases.
“We call it selective breeding, or assisted evolution,” says Nedimyer. “Whatever you wanna call it, the only hope these corals have in the 21st century is for us to do these spawning experiments and try to find some winners that we can turn around and then propagate asexually.”
Be there or be square
Mission: Iconic Reefs partners Mote Marine Laboratory and Coral Restoration Foundation also captured spawning in their Keys’ nurseries, but their events featured the smaller staghorn species. Reef Renewal’s elkhorn experiment, as successful as it was serendipitous, opens a new opportunity. Collecting gametes in the nursery setting is a safer and more practical approach to leveraging the spawning season, when the calendar turns to the fourth night after the full moon in August. At exactly 10:20 p.m.