FLORIDA KEYS — For millennia, the vibrant underwater landscapes off Florida's southern coast have been shaped by two remarkable coral species: elkhorn and staghorn. These branching architects of the sea have built intricate habitats for marine life, from colorful parrotfish to elusive octopuses, enduring since the Ice Age. But now, in a devastating blow linked to extreme ocean heat, scientists have declared them functionally extinct in the region, marking a profound loss for one of the world's most vital ecosystems.
A new study published Thursday in the journal Science delivers this grim assessment, detailing how a historic marine heat wave in 2023 wiped out nearly all colonies of these corals in the Florida Keys and near the Dry Tortugas islands. Researchers, including Ross Cunning, a coral biologist with Chicago’s Shedd Aquarium, surveyed more than 52,000 coral colonies at nearly 400 sites across the state. Their findings reveal mortality rates between 97.8% and 100% for elkhorn and staghorn corals, species that have been cornerstone reef builders for 10,000 years.
“They’ve been the most important reef builders on these reefs for 10,000 years,” Cunning said, emphasizing their role as ecosystem engineers. He added, “The numbers of individuals of these species that remain are now so low that they cannot perform their ecological functions in any meaningful way. This is the functional extinction of two incredibly important ecosystem engineers for coral reefs in Florida.”
The declaration comes amid broader concerns about climate change's impact on marine environments. According to the study, the 2023 heat wave pushed sea surface temperatures to record highs, exceeding levels that had previously caused mass bleaching events. For two to three months, waters off Florida remained hotter than ever recorded, triggering acute heat shock in corals and leading to rapid tissue loss. Divers witnessed the tragedy firsthand, with Cunning recalling, “It was really sad being out there, literally watching them die before our eyes.”
Elkhorn and staghorn corals, which have dominated Caribbean ecosystems for at least 250,000 years, thrive in shallow waters where they form elaborate, branching structures from calcium carbonate skeletons. These invertebrates maintain a symbiotic relationship with photosynthetic algae that provide nourishment and color. Under stress from high temperatures, corals expel the algae, turning white in a process known as bleaching, which leaves them vulnerable to disease and death if conditions persist.
The 2023 event was part of a global pattern, with the National Oceanic and Atmospheric Administration’s Coral Reef Watch reporting that heat stress has affected more than 84% of the world’s reefs in recent years. This widespread collapse is considered the fourth mass bleaching event worldwide. A recent report by 160 scientists from 23 countries suggests global temperatures may have crossed a threshold for irreversible coral decline, potentially the first major tipping point triggered by climate change, alongside issues like ice sheet collapse and ocean current shifts.
While that tipping point remains debated among experts, the Florida die-off strengthens the case. Cunning and his team of 46 researchers note that human-caused global warming, combined with natural variability and reduced cloud cover, likely fueled the prolonged high temperatures. The study highlights how this heat wave surpassed the survival limits of these key species, unlike previous events where some recovery occurred.
Outside experts have expressed shock at the scale of the loss. Laura Mydlarz, a professor at the University of Texas at Arlington who studies coral disease and immunity but was not involved in the research, said the findings were staggering. “I don’t think it had struck me that there was 100% mortality at a lot of these locations,” she said. Mydlarz praised the team's extensive fieldwork, which involved diving and cataloging tens of thousands of colonies.
“The team did an immense amount of work diving and cataloging what they found, which allowed them to produce ‘staggering’ numbers about coral deaths,” Mydlarz added.
These corals are not just aesthetic wonders; they play critical roles in coastal protection and biodiversity. Their structures shield Florida's shores from hurricanes and storms, while supporting fish populations that sustain local fisheries and tourism. As the dead skeletons erode under waves and algae, ecosystems will further destabilize. Mydlarz noted that no other Atlantic species can replicate the 3D lattice these corals provide: “These are the only ones that provide that real sort of 3D structure,” she said, warning that the branches will “start to break down and it’ll start to just crumble.”
This isn't the first such loss in Florida. The rare pillar coral was declared functionally extinct in 2020, making elkhorn and staghorn the second and third species to meet this fate along the Florida Coral Reef. The 2023 heat wave also erased decades of restoration efforts, including projects that had quadrupled coral coverage at some sites over four years through intensive work.
Beyond Florida, the heat wave's effects rippled across the Caribbean. Stacey Williams, scientific director of ISER Caribe, a nonprofit focused on coral restoration, reported that extreme temperatures killed nearly all elkhorn coral in southern Puerto Rico. Subsequent 2024 heat waves hit resilient northern colonies. Her organization is now biobanking coral fragments to preserve genetic material.
Survivors persist in pockets, such as near Miami and Broward County to the north, but researchers doubt they can repopulate southern areas. Ocean currents hinder the drift of reproductive cells southward, Cunning explained. Spawning involves releasing eggs and sperm into the water, but without sufficient numbers or favorable conditions, natural recovery seems unlikely.
Looking ahead, the study suggests importing heat-tolerant corals from elsewhere as a potential solution. Mydlarz mentioned emerging ideas like gene editing to enhance heat resistance, though this remains controversial and requires more research. Last week, the International Union for Conservation of Nature approved exploring gene-editing for wild animal conservation, amid bioethical debates. “That’s on the table because anything is on the table,” Mydlarz said.
The functional extinction of these corals underscores the accelerating threats to marine life from climate change, potentially foreshadowing broader ecosystem collapses. As Florida grapples with this loss, scientists warn that without aggressive action to curb global warming, more tipping points may follow, reshaping oceans and coastlines for generations.