Corals keep cooking in climate-heated seas. These crossbreeds may keep hope alive

Miami Herald

4 days ago

The first-in-the-world experiment began not with a splash, but with a gasp from a respirator.
Neoprene-clad scientists sank to the shallow bottom of Flamingo Reef off Key Biscayne, clutching black milk cartons filled with precious cargo. Inside were a few dozen contraptions that looked like fancy desk toys — round pucks of concrete shielded by a spinning piece of metal resembling the ribs of an umbrella.
Underneath the rotating spines were four thumbnail-sized chunks of coral. Two were the usual suspects for South Florida, hunks of elkhorn coral, and two were newcomers, a crossbreed of Florida elkhorns with their Honduran siblings.
These 'Flonduran' corals are the first ever corals with parents from different countries to be planted in the wild, according to the University of Miami and Florida's Fish and Wildlife Conservation Commission, which approved the experiment.
It's a revolutionary new strategy to save corals as human-caused climate change cranks up the temperature of oceans worldwide. The shallow, turquoise waters of the Caribbean have been hit particularly hard. A 2023 marine heat wave was devastating to the Florida reef tract and many neighboring island nations. Only about 1 in 5 staghorn corals on five major Keys reefs survived the event, the National Oceanic and Atmospheric Administration found.
This pilot project, to share corals throughout the Caribbean and potentially breed new, more resilient varieties that have a chance of surviving the next heat wave, could be a step toward a world where more — but not all — corals survive.
And while the scientific tide appears to have turned on the idea of breaking up corals into smaller pieces, growing them rapidly and planting them on reefs, new research suggests that genetically selecting for stronger coral might still give scientists a chance at restoring some reefs.
'We don't have to plant every single coral on the reef. We just have to plant the next generation. That is the goal of restoration, making these systems self-sustaining,' said Andrew Baker, lead scientist on the experiment and a professor at the University of Miami's Rosenstiel School of Marine, Atmospheric, and Earth Science. 'We're scattering the seeds. We have to wait for the oaks to grow up.'
Baker and his team suited up earlier this month and slipped under the waters of Biscayne Bay to deliver these baby corals to their new home. Twenty feet under, they'll be neighbors with other coral experiments from UM, as well as a forest of colorful soft corals and sponges.
They'll live here for at least a year, with regular checkups from an army of researchers, before they'll get yanked back to the surface for a round of stress tests.
The big question for these tiny animals: Can they take the heat?
From Tela Bay to Biscayne Bay
Climate change is warming the whole planet, but the ocean is absorbing most of that heat. That's bad news for creatures that are sensitive to temperature changes, like corals. When waters get too toasty, corals spit out the algae that live within their skeletons, the stuff they rely on for food and protection from the sun's rays.
Scientists call the ghostly white coral — starving and sunburning — bleached. If a coral stays bleached for too long, it dies.
The oceans are always warming unevenly, with some hot spots turning into coral graveyards and others remaining resilient. That's where Baker and his team got the idea to find the sturdiest survivors and interbreed them with their Florida siblings.
Over the last few years, Baker tried to scoop up corals from Mexico, Belize, the Cayman Islands, the Bahamas and even Cuba, where a massive amount of wild corals perished in 2023 when waters reached 95 degrees. He finally found success in Honduras, where waters are 2 degrees warmer than Florida and soupy with pollution from nearby coastal cities.
And yet, Baker said, the corals in Tela Bay were 'remarkably resilient.'
After a year and a half of permitting delays, Baker and his team managed to successfully airlift a handful of those resilient corals back to Florida. It was a 14-hour journey from sea to lab aquarium, featuring a small plane, a crate of seawater and plenty of bubble wrap.
Once safely in the Sunshine State, the Florida Aquarium interbred the Honduras parents with a stock of Florida elkhorns, creating an army of 'Flonduran' children.
Scientists call this assisted gene flow. In a commentary published Thursday in the journal Science, a team of leading coral scientists argued it may be the best way to save at least some corals.
A reckoning in 2023
In Florida, elkhorn corals in particular are struggling to survive. Only 23 distinct genetic species, out of 153 cataloged before 2023, remain in the wild. The few remaining wild species have all but stopped reproducing in the Keys, scientists say. Some research suggests that, if temperatures continue to rise at the current pace, they could be locally extinct in a decade or two.
'The question is, how do we rescue those corals? They could withstand decades of additional heat stress in other places in the Caribbean,' Baker said. 'If we leave them where they are, they will potentially die off in the next big bleaching event.'
Florida approved the outplanting of the new crossbred coral, but it denied an opportunity to outplant another hybrid coral — Florida corals mixed with corals from Curaçao — a few years back. Those offspring are still stored at a research aquarium in Florida. That's because the Honduras corals are genetic siblings to Florida corals, while Curaçao corals are further removed, like cousins once or twice removed. Corals from other locations, like Hawaii, are essentially strangers, scientists say.
That leaves an increasingly shrinking pot of corals for Florida to choose from, if this is a strategy the state continues to pursue.
While some may look at this strategy of swapping corals around a small region as the whole ocean cooks more like rearranging the deck chairs on the Titanic, Baker said he prefers to see it as buying time.
'We need to buy time for as many species to thrive while we as a society figure out whatever the solution is going to be to climate change,' he said. 'It's an inconvenient truth that while this method can be used to help corals in some parts of the region, it's not a solution for all.'
The science backs him up. An increasing amount of research has soured on the once very popular idea of rescuing dying reefs by choosing fast-growing species, breaking them up into tiny fragments and planting those regrown pieces on reefs.
'Coral restoration has been a very hot and sexy topic for years. Because of the growing recognition in the coral reef science community that restoring coral is difficult, the research is losing a little bit of momentum,' said Giovanni Strona, a researcher at the European Commission who has studied tropical reefs since 2008.
In a paper published in April, Strona and his team argued that restoration only works under narrow circumstances. Replanting a huge number of genetic copies of one type of coral is like building an entire city with only one-bedroom apartments. It's not enough to attract the diverse, healthy ecosystem needed to survive disease, predators or climate change.
'You need to create a reef that's as diverse as the original one. Of course, having something is better than having nothing,' Strona said.
It's also simply not happening fast enough. He compared replanting new corals to reforestation projects happening all around the world; they're not keeping up with the global loss of forests — at all. In total, he found, only a few square meters of reef around the world have been restored in recent years.
'It's not about restoring even three soccer fields. We're really talking about very tiny islands,' he said.
However, the newest wave of coral research suggests that genetically selecting for stronger, better corals — including interbreeding via assisted gene flow — could still be a visible solution to keep some reefs in selected areas viable. A paper published last year found that lab-reared corals survived the 2023 Caribbean marine heat wave better than nursery-grown or native corals.
But in some places, it may already be too late.
'Elkhorn and staghorn corals in some of the region's warmest areas, off the south coast of Cuba, were exposed to unprecedented heat stress during the 2023 bleaching event and have experienced major losses. It is not clear whether these reefs can recover through immigration of even more thermally tolerant genets from elsewhere because these reefs are among the warmest in the region,' the authors wrote in the Science commentary.