Two essential coral species are now functionally extinct — but should we give up hope?

In October 2022, PhD students Jenna Dilworth and Maya Gomez found themselves in 20 feet of water on a reef in the Lower Florida Keys, several thousand miles from their lab at the USC Dornsife College of Letters, Arts and Sciences. They were there to transplant 200 staghorn corals at two reef sites as part of a collaborative experiment examining coral growth in different environments with long-time partner Mote Marine Laboratory. As the team completed their task and headed home, they couldn’t have known the dark fate awaiting the newly transplanted corals — and, in fact, the entire reef system.

The team monitored the sites regularly over the ensuing months, and as late as June 2023, the corals appeared completely healthy. But, only one month later, that all changed with widespread bleaching devastating the entire ecosystem. By November, only three corals in their Lower Keys experiment clung to life, victims of a record-setting marine heatwave that sparked the ninth mass coral bleaching event to strike Florida’s coral reefs.

Members of USC Dornsife’s Cnidarian Evolutionary Ecology Lab (CEE Lab) — led by Associate Professor of Biological Sciences Carly Kenkel — observed similar patterns elsewhere. In June 2023, Gomez and researchers from other institutions transplanted more than 500 staghorn corals across three sites in Dry Tortugas National Park, about 70 miles west of Key West, as part of a study testing coral tolerance to elevated water temperatures. By October, the team estimated that all but the barest fraction — specifically, 99.9% — of those corals had died.

Members of the lab dutifully contributed mortality data for the corals they outplanted, as did researchers working at hundreds of other reef sites throughout Florida who observed similar trends. The combined data revealed that, tragically, two ecologically vital but critically endangered reef-building species, staghorn and elkhorn corals, were all but completely wiped out.

The dangers of coral bleaching

Coral bleaching occurs as a result of environmental stress. This was the case in 2023, when sea surface temperatures spiked to their highest levels in more than 150 years. Healthy corals have a close partnership with algae that live in their tissue. These algae provide the coral with food through photosynthesis, giving reefs their vivid colors. In return, the corals protect the algae and offer nutrients. But when water temperatures get too high — even a degree or two above normal — the algae produce toxins, forcing the coral to expel them. What’s left is the pale, “bleached” skeleton of the coral, now far more vulnerable to disease and death.

Bleaching events pose threats not only to coral reefs but also to entire marine ecosystems, disrupting the services that healthy reefs provide. Coral reefs serve as critical habitat to about a quarter of all marine life. They also protect coastal ecosystems from storm surge and erosion by acting as buffers against intense waves. The National Oceanic and Atmospheric Administration reports that, when healthy, coral reefs can absorb up to 97% of a wave’s energy, protecting shorelines and coastal communities and properties.

Bleaching renders key corals functionally extinct

The 2023 bleaching event resulted in a near-total loss of elkhorn and staghorn coral species in the Keys and Dry Tortugas, signaling their “functional extinction.” This means that there are now too few to fulfill their ecological role, thereby disrupting the structure and function of the surrounding reef community. While these corals have historically been ecologically valuable in the Florida Keys and are often used in coral restoration projects due to their fast growth and reef-building abilities, they are also more sensitive to temperature changes.

Gomez, who returned to the transplant sites several months after the initial onset of the heatwave, witnessed firsthand the transformation from vibrant, orange-brown colored corals to bone-white, bleached reefs. “I had the overwhelming, I-can’t-believe-this-is-happening, tragic feeling that I recognized as shock. And heartbreak. And dread.”

The work that Gomez and so many other passionate researchers dedicated years of time and energy to was lost in a matter of weeks. But the tragedy offers lessons, and restoration practitioners are reconsidering how best to support these essential coral species in the face of further environmental threats.

Science offers path to saving corals from extinction

Dilworth says that instead of continuing to transplant corals and hoping that they will survive amid a rapidly changing climate, researchers must adopt more advanced strategies to ensure the corals are more resistant to thermal challenges. Along with farming corals, practitioners are also adopting practices such as assisted gene flow, breeding Caribbean corals with those from other countries to increase their genetic diversity and improve their ability to adapt to changing conditions. Additionally, encouraging juvenile corals to cultivate a symbiotic relationship with thermally tolerant algae — those that can better handle higher temperatures — can enhance their tolerance to bleaching. Introducing bacteria that help increase resistance to environmental stress to the corals’ microbial communities and microfragmentation (cutting farmed corals into smaller pieces) also show promise.

But the researchers emphasize that implementing new restoration strategies won’t be enough, and addressing the root cause of this destruction — climate change — is imperative. “Climate change is the reason all this is happening, and if we don’t address that, nothing we do is going to fix this,” says Dilworth. “Our work is a stopgap. We’re buying time at best, and we need to address the underlying cause, or none of this will help in the long term.”

Coral researchers remain hopeful despite climate change

Yet, despite the direness of the situation in the Caribbean, the fight against climate change isn’t over. “Never in my life did I think I would be happy to see a bleached coral,” says Gomez. “But in this moment, the bleached corals gave me hope. They weren’t dead yet. They could recover. And I hold on to that. And I hold on to the resilience of the researchers and restoration practitioners around me and the work that they are doing to respond and recover.”

By working together and implementing thoughtful mitigation practices, restoration practitioners and researchers aim to secure a more sustainable future not only for these essential reefs but also for the planet as a whole. From hosting workshops on the future of coral reef conservation to sharing policy briefs with Caribbean policymakers and conducting new studies on coral health and biology, the CEE lab and its partners in coral restoration are making meaningful progress toward healthier reef ecosystems. As Gomez notes, it is the resilience of researchers and restoration practitioners that will sustain us through this climate crisis and those yet to come.