In a lab at the University of Miami, there are tanks of knobby, tan-colored corals from Florida, Honduras and the Cayman Islands. They've been drafted into a sort of coral Olympics, as scientists look for the ones that can best survive increasingly hotter ocean temperatures driven by climate change.
Over the past two years, 80% of the world's reefs saw dangerous levels of heat, which causes coral to bleach, turning a ghostly white color.
Florida's reefs are on the front line of the crisis. Corals there bleached again this summer, and already more than 90% of the living coral off the Florida Keys has died. So for decades, the focus has been on restoring reefs by growing and planting coral in the ocean, much like replanting a forest.
It's one of the largest coral restoration efforts in the world. But after much of the restored coral near Florida died in the recent marine heat wave, restoration groups have had to overhaul their strategy.
Scientists are now trying to breed corals that can tolerate heat better, speeding up the natural process of evolution. Florida coral has been crossbred with coral from Honduras, creating what researchers call "Flonduran" corals. For the first time in the U.S., those coral babies have been put into the wild in a controlled trial.
Researchers are hopeful that Florida will develop a strategy that can be copied around the world to help ailing reefs. The question is whether it can be scaled up enough to make a meaningful difference — or whether temperatures will simply get too hot for corals.
"Florida occupies an unenviable position right now of having the world's coral reef eyes on it, because of the state of the resource and the fact that we are so dependent on it," says Andrew Baker, professor of marine biology and ecology at the University of Miami's Rosenstiel School of Marine, Atmospheric, and Earth Science. "So if anywhere in the world is going to try to do something a little out of the box, it's Florida."
Florida's ghost reefs
Ken Nedimyer is one of the few people who know what Florida's reefs used to look like. He's been diving off the Florida Keys for more than 50 years and says the fields of coral used to be so dense, you couldn't swim through them. They were filled with delicate, branching corals known as elkhorn coral and staghorn coral, which resemble antlers.
"Just incredible," says Nedimyer, technical director of Reef Renewal USA, a coral restoration group. "You could look left and right and forward and you couldn't see anything but that. These great big branches would go out 6 feet from the base, all interlaced into each other."
Now, those reefs are just a memory.
"The elkhorn coral, except for maybe one or two reefs off Key Largo, it's all dead," Nedimyer says. "All completely dead."
Florida's coral reefs have declined for decades because of many threats, including poor water quality and damage from boating and human impacts. The waterborne Stony Coral Tissue Loss Disease spread as well, which kills some species of coral.
In 2019, the National Oceanic and Atmospheric Administration (NOAA) started the Mission Iconic Reefs program, setting a goal to restore coral cover from just 2% to 25% on seven reefs off the Florida Keys. NOAA works with nonprofits like Nedimyer's, which grow coral in nurseries and plant it on the reefs. In the first five years, about 40,000 corals were planted. To reach its goal, NOAA estimates it will need 5 million corals.
Those coral restoration groups largely focused on elkhorn and staghorn corals, which grow quickly and provide key habitat for fish and marine life.
Those branching corals are also the most sensitive to heat.
Marine heat wave hits
In 2023, the ocean around Florida started heating up far earlier than normal. Water temperatures reached well into the 90s, hotter than branching corals can withstand. Heat disrupts a key partnership that corals depend on. They rely on algae that live inside their tissue, which use sunlight to make food for the coral. But when corals get heat-stressed, those algae get ejected, which is why corals turn white.
Bleaching doesn't necessarily kill coral, but after weeks of heat stress, corals can starve and become susceptible to other stressors. On some reefs that had been restored, NOAA found that 95% of the elkhorn coral had died.
"It looked like there was a snowstorm, just white coral everywhere," Nedimyer says. "Two weeks later and you go out there and they're all gray and dead. And it was so hard to see. Twenty years of hard work gone in one week."
The extreme temperatures were also a glimpse of the future. Marine heat waves are becoming more common, and coral reefs are among the most vulnerable ecosystems in the world to climate change. Studies show that if climate change stays on the current path, surpassing 3.6 degrees Fahrenheit (2 degrees Celsius) by 2100, 99% of the world's coral will be lost.
It was a moment of reckoning for Florida's coral restoration community.
"Just knowing what it's going to look like really helped us to focus on how we're going to prepare for it," Nedimyer says. "What we were doing was not going to work."
Nedimyer's team is now focusing more on the coral species that survived the 2023 heat wave better. The large, round corals, known as brain and boulder corals, are slower growing but fare better in higher temperatures. He's developing plans for an onshore coral nursery, so larger amounts of corals can be grown in tanks, before being put into the ocean. Still, in a hotter future, he knows Florida's reef may need even tougher corals.
Crossbreeding corals
At the University of Miami, the tiny "Flonduran" elkhorn corals are about the size of a quarter, the result of years of planning and international logistics. Their Honduran parents came from a reef known to experience tough conditions for elkhorn coral — high temperatures and murky water.
"The reef in Tela, Honduras, routinely experiences temperatures that are about 1 1/2 to 2 degrees warmer than Florida's warmest temperatures and those are about the same kind of conditions that we saw in Florida in the 2023 bleaching event," says Baker of the University of Miami.
The hope is that the Honduran coral can pass on their ability to withstand heat in their genetics. It's blended with the genetics of Florida's coral, since they have their own strengths suited to the local environment. The technique is known as "assisted gene flow," a way of speeding up the process of adaptation within a species.
Baker worked with a nonprofit in Honduras, Tela Marine, and The Florida Aquarium to bring the coral to the U.S. This summer, their young were planted on a reef off Miami to test how they grow on Florida's reefs.
"So far so good, they're all doing great," Baker says. "But we're under heat stress, so we are actually very interested to see what happens in this pretty severe bleaching event."
It's the first time that state regulators have permitted an internationally bred coral in U.S. waters. Regulators with the Florida Fish and Wildlife Conservation Commission considered the risks of the introduction, given the damage worldwide that invasive species have caused through accidental introductions. Researchers have also crossbred Florida corals with ones from Curaçao, but regulators did not permit them to be planted in the ocean. While the parent corals are the same species, they determined the populations were too genetically different to be released in the wild.
"Rightly, we've been very protective of environments to try to prevent introductions when we can," Baker says. "I think what's changed on Florida's coral reef is that the conditions are no longer suited for the coral we have here. So we really have to think about bringing in new diversity."
Still, Baker says the work isn't about creating a "super coral." Corals that are all-stars at dealing with heat may struggle to deal with other threats, like disease. Reefs need a mix of genetic diversity to withstand what nature throws their way.
"What we want is lots of different super corals that were generated in different ways so that we don't have all our eggs in one basket," Baker says.
Baker's lab is also looking at breeding corals from the Cayman Islands. He's hopeful that countries around the Caribbean will benefit from their research, an incentive for sharing coral samples. But he says the lengthy international permitting process makes moving coral samples challenging. Countries like the Bahamas have also passed laws limiting the export of their biological materials, after a historical legacy of richer countries extracting and patenting discoveries made in developing nations.
Still, Baker says he's hopeful that coral conservation work can be reciprocal for all the countries involved, given the dire situation that all reefs face.
"It's not all just about Florida," Baker says. "I think we have a role to play here in Miami where we have the resources to help other countries generate varieties of corals that will do well in their countries."
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