For coral, sex is a numbers game.
Every summer, days after the July full moon, elkhorn and staghorn corals release eggs and sperm into the water in a carefully timed event called mass spawning.
When enough mature colonies spawn together, their gametes can meet and mix in the waves, fertilize each other and hopefully settle back onto reefs to grow into new colonies.
In the Keys, that ancient biological process has become surprisingly difficult.
Elkhorn and staghorn corals, species which used to dominate Florida and Caribbean reefs, have declined so dramatically that they are now considered “functionally extinct” across much of Florida’s coral reef. Remaining colonies which are large enough and healthy enough to spawn are often too few and far apart for their eggs and sperm to reliably find each other.
It’s a little like having all the eligible singles in neighboring towns hoping to meet – but nobody having a car, phone or internet to help them connect.
Rather than bet on poor odds for this summer’s spawn, Florida’s coral restoration community is coming together to play matchmaker.
“The temperature is rising so fast, and the population is functionally extinct — it’s basically gone — so there’s no chance it can recover naturally through natural selection,” said Reef Renewal USA’s technical director Ken Nedimyer, who will host the spawning event.
The timing couldn’t be more important.
The deadly 2023 marine heat wave devastated Florida reefs. Anticipating this summer’s ocean temperatures again running dangerously high, scientists are racing to beat the heat and preserve genetic material from some of Florida’s hardiest remaining corals. One way to do that is in living coral biobanks. The other, which is what this year’s big spawning event focused on, endeavors to create new genetic mixes from remaining coral genotypes that will hopefully be more resilient to ocean conditions in the future. These offspring could, in best cases, be replanted on degraded reefs in a few years.
Reef Renewal’s offshore nursery in Tavernier is home to an unusually large collection of mature elkhorn and staghorn corals that were used for the spawning effort. These were not chosen at random. During the 2023 heat wave, Reef Renewal left many of its mature corals in its Tavernier nursery rather than evacuating them all to land-based tanks. The heatwave effectively created a massive, uncontrolled experiment for those remaining corals. Some colonies bleached badly; others barely did.
“We scored every coral during that summer, and we took photo records,” Nedimyer said. “Now we know which ones bleach and which ones don’t, and we focus most effort on the ones that don’t.”
The scientific and restoration goal now is to see whether those traits can be passed to the next generation — and potentially strengthened through selective breeding. Scientists will use some of those 2023 survivors for this year’s spawning efforts, capitalizing on the valuable opportunity to identify and propagate corals that demonstrated unusual heat tolerance.
And so, the matchmaking began as researchers endeavored to help the corals “get it on.”
Before the big event, Nedimyer set aside 100 large elkhorn colonies representing 48 unique genotypes for spawning, including 22 genotypes identified as having exceptional or good heat resistance. Another 200-plus large and extra-large elkhorns in the Reef Renewal nursery represent 68 additional unique genotypes that could potentially spawn, he said. Finally, hundreds of mature staghorn colonies representing 18 unique genotypes were also selected for the effort.
Nedimyer’s decades of experience paid off: “The corals are spawning like gangbusters,” he told the Weekly on Aug. 1. On Aug. 4, Nedimyer sent another update: “The corals went wild last night. Best night ever.”
Over nine nights of spawning, dive teams collected coral eggs and sperm from the Reef Renewal nursery and raced them back to a temporary laboratory at John Pennekamp Coral Reef State Park. Scientists took over, working around the clock to separate the gametes, create genetic crosses and begin rearing the resulting larvae. Some sperm will also be cryopreserved to create a genetic bank for future restoration efforts.
The 2026 effort brings together 11 leading coral conservation and restoration organizations, funded through NOAA’s Re5 initiative. Reef Renewal has mature corals and nursery infrastructure, while partners including Mote Marine Laboratory, the Florida Aquarium, the University of Miami, SECORE International, the Reef Institute and others bring expertise in fertilization, larval development, cryopreservation and growing young corals.
“It’s the perfect storm.” Nedimyer said. “We’ve all come together at the right time. Best of the best working together.”
For Nedimyer and his colleagues, the summer matchmaking may be coming to a close, but the real work is just beginning. The baby corals produced through the effort will be distributed to partner facilities for growout and, hopefully, eventual restoration back on the reef. These tiny recruits represent something Florida’s reefs have been missing for years now: new genetic combinations, more corals and renewed hope.
