'Functionally extinct': After 10,000 years on the Florida coast, two key corals are dead

Since the Ice Age, elkhorn and staghorn corals off Florida’s southern coast have been stacking their skeletons into elaborate, branching homes for parrotfish, eels and octopuses.

“They’ve been the most important reef builders on these reefs for 10,000 years,” said Ross Cunning, a coral biologist with Chicago’s Shedd Aquarium.

But researchers are using stark, new language to describe the status of the two species in Florida: functionally extinct.

“The numbers of individuals of these species that remain are now so low that they cannot perform their ecological functions in any meaningful way,” Cunning said. “This is the functional extinction of two incredibly important ecosystem engineers for coral reefs in Florida.”

Cunning and a team of 46 other researchers published a grim study in the journal Science on Thursday that assesses the damage caused by a historic 2023 marine heat wave in Florida. The findings are essentially an announcement that the two coral species have disappeared there because of extreme ocean temperatures.

The researchers determined that between 97.8% and 100% of these species’ colonies have died in the Florida Keys and near the Dry Tortugas islands. The findings came after divers from institutions across the state visited more than 52,000 coral colonies at nearly 400 sites.

“For the first time, we’ve now just had a heat wave that was so extreme it has surpassed the limits of some of the most ecologically important species in an entire ecosystem to survive,” Cunning said.

Corals support fish and other aquatic species that help feed coastal communities and attract tourists. Importantly, they also provide a natural barrier for Florida’s coastline during hurricanes and other storms. As the dead structures of elkhorn and staghorn coral get broken down by waves and algae, their absence will further scramble the state’s coastal ecosystems.

Heat stress has affected more than 84% of the world’s reefs in the last few years, according to the National Oceanic and Atmospheric Administration’s Coral Reef Watch — a collapse so widespread that it is considered the world’s fourth mass bleaching event.

In a recent report, 160 scientists from 23 countries suggested that global temperatures have crossed a threshold for coral’s irreversible decline, the first key global “tipping point” triggered by climate change. Other tipping points include the collapse of ice sheets and rapid shifts to ocean currents.

Although that conclusion remains a subject of debate, the die-off of elkhorn and staghorn corals in Florida could bolster the argument.

Elkhorn and staghorn coral have dominated coastal Caribbean ecosystems for at least 250,000 years. The two species grow in branching colonies in shallow water. The invertebrate animals develop a hard skeleton of calcium carbonate, which forms the structure of living reefs.

The animals have a symbiotic relationship with photosynthetic algae that lives within the corals’ cells, providing food and giving them color. When corals are stressed, they release the algae, turn white and become more prone to disease. Such bleaching is a signal that corals’ health is endangered. If temperatures remain too high, they eventually die.

In 2023, sea surface temperatures reached record highs across the globe and remained that way for more than a year, likely driven by a combination of human-caused global warming, natural variability and a decrease in cloud cover (though researchers are still working to parse these factors).

In Florida, water temperatures for two to three months exceeded the highs that had caused mass bleaching in past years, the new study says, which sent many corals into acute heat shock and caused rapid tissue loss.

“It was really sad being out there, literally watching them die before our eyes,” Cunning said.

Other coral species have rebounded, but elkhorn and staghorn are now the second and third to be declared functionally extinct along the Florida Coral Reef; the rare pillar coral was declared functionally extinct in 2020.

The extent of damage documented in the new Science paper has surprised some outside researchers.

“I don’t think it had struck me that there was 100% mortality at a lot of these locations,” said Laura Mydlarz, a professor at the University of Texas at Arlington who researches coral disease and immunity but was not involved in the study.

Mydlarz said the team did an immense amount of work diving and cataloging what they found, which allowed them to produce “staggering” numbers about coral deaths.

As for structures left behind by the dead corals, she said, “all of those little branches will start to break down and it’ll start to just crumble.”

Mydlarz added that no other Atlantic species grows as large or forms such an extensive lattice.

“These are the only ones that provide that real sort of 3D structure,” she said.

There are some pockets of coral survivors, including remnants to the north, near Miami and Broward County. However, the researchers think there’s little hope they will spur a comeback. Corals spawn by releasing eggs and sperm to mix in the water and settle, but ocean currents will prevent those reproductive cells from drifting south, Cunning said.

While this study focused on Florida, the 2023 heat wave has had a dramatic effect on elkhorn and staghorn corals across the Caribbean, outside researchers said.

Stacey Williams, the scientific director of ISER Caribe, a nonprofit research and education organization focused on coral restoration, said extreme temperatures killed nearly all of the elkhorn coral in southern Puerto Rico. Then, heat waves in 2024 devastated some northern colonies that had demonstrated more resilience. Her organization is focused on “biobanking coral species,” Williams said, by collecting living fragments.

In Florida, the 2023 heat wave erased decades of restoration projects for elkhorn and staghorn corals, including at sites where four years of intensive work had increased coral coverage by about four times.

The study says importing heat-tolerant corals from outside the state might be the only path forward.

In the future, Mydlarz said, scientists could consider editing corals’ genes to make them more heat resistant, but the idea would require more research and scrutiny.

The International Union for Conservation of Nature last week approved further exploration of whether gene-editing tools can be used to conserve wild animal populations — a fiercely debated topic because of its bioethical concerns.

“That’s on the table because anything is on the table,” Mydlarz said.

This article was originally published on NBCNews.com