In 2023 and 2024, the Florida Keys experienced a marine heat wave that caused a 98% to 100% catastrophic mortality in staghorn and elkhorn corals.
The corals are so named because the animals grow into branches that can look like the horns of their namesakes. And they are essential in helping build coral reef structures.
"They're the predominant branching coral species that we see in Florida," according to Hanna Koch, PhD., with the artificial reef program in Monroe County.
Or used to be. A 2025 research paper, lead by the National Oceanic and Atmospheric Administrations' Coral Reef Conservation Program and Chicago's Shed Aquarium, classified both corals in the lower Florida Keys as functionally extinct due to the marine heat wave.
Functional extinction means that the few animals that are still around are too spread out to do what they’re supposed to do: provide habitat for marine life and make baby corals. The step after functional extinction is global extinction.
The classification wasn't a surprise to Dr. Koch. She'd seen it first hand. It also heralds a changing underwater landscape for her.
"They used to be highly abundant on our reef systems. And they've been declining for decades now. And so we're seeing reef flattening across the Caribbean. We're seeing declines in fish density and diversity, likely connected to the loss of structural complexity and vertical relief, with, with the flattening of our reefs," said Koch.
The lack of vertical relief can mean less coastal protection from waves and storms.
But all is not lost, yet.
Scientists like Doctor Cindy Lewis, director of the Florida Keys Marine Laboratory, acted quickly when the marine heat wave hit.
"There was a mass emergency rescue of as many of the corals as they possibly could bring in from their offshore nurseries that were in the ocean to get them out of the hot water. And they brought them here. We literally had every single tank table and, and container that could hold seawater full of corals here."
The Keys Marine Laboratory, as part of the Florida Institute of Oceanography, houses infrastructure for educational opportunities at the college and research levels.
Lewis said, they are also a hub for coral restoration in the Florida Keys because of their extensive temperature-controlled seawater systems.
The rescue of the staghorn and elkhorn corals was important for Lewis personally. She saw one of her favorite corals disappear from the Florida underwater ecosystem.
"My passion is, specifically, the pillar coral, Dendrogyra cylindrus. It is now virtually extinct on Florida's reef because of all of the impacts of warming seas as well as disease."
However, scientists are working to mitigate these losses, by restoring corals to their original habitat, and by helping corals develop resiliency through gene banks.
"We can still find pillar coral that have been genetically banked in a living genetic bank, starting at Florida Aquarium, in Tampa," said Lewis. "We have far more than 500 chunks of pillar coral, and now they're distributed across multiple agencies, multiple land-based nurseries, so they can be protected. And they are actually spawning in captivity and producing brand new baby corals."
Another Florida-situated gene bank is at Mote Marine Laboratory in Sarasota.
"So, coral restoration is the process of growing corals either in the water and land-based nursery. So, think about these trees with a bunch of corals hanging on them," said Dr. Sarah Williams a staff scientist with Mote's Coral Health and Disease. "So we grow out these corals and we do something called out-planting where we put them back out on the reef. Just kind of how you plan to garden. We also outplant corals onto the reef to restore certain areas and, kind of hopefully, bring back that living coral cover."
Williams already had planted staghorn coral babies in the Florida Keys when the 2023-24 marine heat wave hit.
"We lost about 95% of our output in the lower Florida Keys to the bleaching event," said Williams.
Of the 5% that survived, she hopes they can learn from them. "What made them special? What made them stronger and more resistant to this heat stress? And a couple things that we've learned so far is that it's the corals produced through assisted sexual reproduction" that had the most success.
Assisted sexual reproduction is when corals are bred in the lab to make new coral babies.
"Those babies are the ones that actually had the highest survival during the 2023 bleaching events," said Williams.
In addition to spawning corals in the lab, Dr. Lewis said they are able to grow resilient babies from using pieces from surviving coral.
"They can cut it up into smaller fragments or even micro fragments the size of my finger," she said. "They might take eight, 10 or 15 of these fragments, put them in an area the size of a dinner plate under water. In five or six years they can grow together. So, they've essentially formed a single coral head. Now this coral thinks it's the size of a 50- or 75-year-old coral, and it's reproductive, it can spawn."
Dr. Lewis said breeding the survivor coral is key.
"Now, if you take broodstock, wild coral, a single coral colony, one here and one there, different genetic structures, they're already been through the gamut of hot summers and cold winters and hurricanes and the nutrients and everything, " she said. "They've already been toughened. There's something about them that is more resilient and resistant to the changes that they've seen."
"And now you let them spawn naturally, in the hope that by crossing them, letting these little tiny polyps settle and grow (very slowly,) perhaps those brand new corals are truly the new coral recruits."
While using coral fragments takes less time than achieving a spawning in the lab, both methods are necessary, according to Dr. Lewis.
No matter how they're reproduced, Dr. Williams hopes the new baby corals will better handle the increasing heat and stress.
"You know, part of our job is trying to make it so that we don't have a job to do anymore. That's that's the dream, right? You know, once we have a thriving Florida coral reefs, we can hopefully all retire or maybe just ask different questions."
Dispatches from Kimberly's Reef are part of a WGCU documentary project called "Kimberly's Reef: An Underwater Living Laboratory" due out in September 2026. Watch previous dispatches.
The Kimberly's Reef documentary project is generously and partially funded by Bodil and George Gellman.
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