These Coral Have a Unique Survival Strategy That Lets Them Recover After Deadly Warming Events

Scientists have found that some coral species are able to recover from harmful warming events through a unique survival strategy that was previously known only in extinct fossil corals.

Diego Kersting and Cristina Linares, who are from the Freie Universität Berlin and Spain's University of Barcelona, respectively, uncovered the first living evidence of this strategy—known as "rejuvenescence"—among corals in the Mediterranean Sea, according to a study published in the journal Science Advances.

The findings represent some rare good news for corals around the world, which are facing numerous severe threats—most notably, climate change. As the world warms, so does the average temperature of the planet's oceans, making damaging events more likely.

"The main threats are climate change, overfishing, pollution and coastal urbanization," Kersting told Newsweek. "But currently, climate change is probably the one causing the most coral cover declines. Warming stresses corals up to a point that may cause death. Some corals bleach before dying. Other corals do not bleach but die directly, like Cladocora caespitosa, the coral in this study."

He went on, "Our findings are important because this survival strategy was only known from fossil corals that existed hundreds of millions of years ago. It is the first time that it is found in a living coral. This strategy is allowing this coral to slow down its decline caused by warming-related mortalities."

For their research, Kersting and Linares monitored 243 colonies of the endangered reef-building coral Cladocora caespitosa in Spain's Columbretes Islands Marine Reserve over 16 years, starting in 2002. They examined the colonies in the summer months when deadly warming events usually take place, documenting the damage with photographs and sketches.

Aside from having a heavily urbanized coastline, the Mediterranean region is under significant stress from climate change and related summer heat waves. These heat waves subject corals to significant stress and starvation, which can lead to the death of colonies. For example, a summer heat wave in 2003 led to a 25 percent reduction in coral cover in the Columbretes Islands, according to the researchers.

The monitoring revealed that Cladocora caespitosa in the Mediterranean uses rejuvenescence to cope with warming events. This process involves the polyps—or the individual coral animals in a colony—shrinking inward and abandoning their skeletons during warm periods, before rejuvenating at a later point.

"What happens is that some polyps in a coral colony—sometimes just one—reduces drastically its dimensions and partially retreats from its skeleton," Kersting said. "Once the stressful event is over, the shrunken or rejuvenated polyp recovers its size and builds up a new skeleton. Eventually, it begins to reproduce itself through budding and begins to cover the dead colony surfaces."

He continued, "The results were very surprising because I started to observe colonies that were dead years ago, that were showing living parts many years after their death. I was even more surprised when I realized that this process was only known from extinct fossil corals."

The only known fossils that displayed this ability lived hundreds of millions of years ago, according to the researchers. This survival strategy may have been overlooked until now because colony recovery takes a long time, so only long-term monitoring studies can reveal the process. Over a decade, 13 percent of the colonies affected by warming in the study experienced a full recovery.

"[Rejuvenescence] is an effective, and unexpected, survival strategy that is allowing warming-impacted corals to recover after warming-induced mortalities," Kersting said. "This is good news! Thanks to our findings, we know now that some corals are able to recover, but unfortunately this is not enough in the current climate change context."

Corals have been found to adopt various survival strategies to deal with their changing environment. However, the researchers say that environmental change is happening too fast, with increasing warming and the higher frequency of marine heat waves, and corals have little time to adapt. This highlights the urgent need to combat climate change, they say.

"We think that these survival strategies open a narrow window of opportunity for at least some coral species, but only in a scenario of decreasing greenhouse gases emissions and slowed warming rates," Kersting said.

Mark Hay, a professor from the School of Biology and Aquatic Chemical Ecology Center at Georgia Institute of Technology, who was not involved in the study, echoed Kersting's sentiments.

"Finding this mechanism of persistence in both fossil and living corals provides a bit more hope that some corals may persist despite dramatic recent declines," he told Newsweek. "This finding that a modern coral can sometimes recover from severe bleaching via a transitory resistant stage, coupled with evidence of recovery in the fossil record, offers hope that some corals may be more resilient than thought."

"The challenge will be whether this mechanism of the coral buying some 'insurance' against extinction due to periodic stresses will be enough to allow long-term persistence since small, isolated individuals are less likely to survive than larger, abundant ones," he said.

Mark Warner, a professor from the School of Marine Science and Policy at the University of Delaware, who was also not involved in the study, described the paper as "interesting" but said he had several issues with its results raising much 'hope' for corals, as the title implies, especially for reef building corals.

"The paper is unique in how this particular coral species manages to regrow new corallites from within the original skeleton," he told Newsweek. "However, and as the authors correctly cite, a few species of tropical reef building corals have also showed a similar 'phoenix' effect of retaining just enough living tissue to start to grow again and in some cases very quickly and successfully to outcompete overgrowing algae."

"These tropical examples are the result of different processes, such as coral tissue sinking down deeply into the porous skeleton to survive the warming," he said.

Warner also noted how recovery in Cladocora caespitosa took a long time, with annual rates of just over four percent.

"Only 13 percent of the colonies recovered by 90 percent and here is the catch: it took over 10 years," he said. "That can be a very long time on a tropical coral reef, and especially one that is already disturbed by overfishing and other human influence that allows algae and sponges and other organisms to grab the open space and overgrow the dead coral skeletons."

Furthermore, he pointed out that the species had found an interesting way to rebuild itself that may not exist for many tropical reef corals which are incredibly susceptible to high temperature stress.

"I agree that this strategy is a neat piece of the puzzle for how this particular coral and, perhaps its relatives that had a similar morphology, may have made it through previous periods of temperature stress, but I'm not convinced that we will find many parallels to tropical reef corals that are already under the incredible strains of climate change," he said.

Recently, researchers shed light on another type of surprisingly resilient coral when they identified so-called "extreme corals" for the first time in mangrove lagoons around Australia's Great Barrier Reef. This discovery could have significant implications for our attempts to save stricken coral reefs.

"The mangrove lagoons hold a potential stock of stress-hardened corals that could be considered for reef intervention activities, such as local-scale coral transplantation to degraded reef areas," Emma Camp, an author of a University of Technology Sydney study describing the extreme corals, previously told Newsweek. "Further work is required to assess how this could be scaled up."

The importance of coral reefs to the world's oceans is hard to overstate. While they cover less than 1 percent of the Earth's surface, they are home to around 25 percent of known marine life and host the highest biodiversity of any ecosystem globally.

They also play a hugely influential role in the world economy, with an estimated 500 million people directly relying on coral reefs for food, resources and livelihoods.

This article was updated to include additional comments from Mark Hay.

coral, Cladocora caespitosa
Polyps of the coral Cladocora caespitosa. Diego K. Kersting