The Mediterranean pillow coral, an endangered stony coral species that builds the only true reefs in the Mediterranean Sea, weathers deadly warming events by employing a novel survival strategy, according to a new study published in the October 11 issue of Science Advances.
During periods of warming-induced stress, the coral polyps shrink inward, partially abandoning their skeleton, only to reemerge years later and revitalize dead colonies.
While this "rejuvenescence" phenomenon has been previously described in fossil corals, it had never been observed in living ones — perhaps because the recovery process is slow and afterward the colonies appear misleadingly undisturbed. This strategy may provide a narrow window of opportunity for some corals, at least, to cope with warming caused by climate change.
Diego Kersting, a researcher at Freie University in Berlin and the first author of the study, and co-author Christina Linares, a professor at the University of Barcelona, weren't expecting to discover secret coral defenses when they began observing 243 Mediterranean coral colonies on a permanent transect (a fixed path along which scientists count and record organisms) near the Columbretes Islands in 2002. They just wanted to study the organisms' long-term response to an onslaught of summer heat waves.
"The description of slow processes in slow-growing, long-lived organisms like these corals needs many years of precise field observations," said Kersting. "The real-life history of each colony can only be known when working on the long-term with permanent transects or by analyzing inner colony and corallite structures."
For 16 years, Kersting and Linares visited the same pillow coral (Cladocora caespitosa) colonies after the summer months, when deadly warming events left the corals reeling. They recorded the damage with photographs and sketches, inspecting extreme close-up shots to identify surviving polyps.
Although the researchers detected no signs of life in the first years following the colonies' destruction, they eventually observed recoveries in 38% of warming-impacted colonies, with polyps growing to their original size and budding over dead neighbors. After a decade, near-full recoveries were observed in 13% of colonies.
As seen in fossil corals, each surviving polyp's metabolic activity appeared to decrease during periods of warming-induced stress. This caused the polyp to shrink drastically, partially retreating from inner skeletal structures and forming a new calyx (the protective cup within which the polyp sits) while staying connected to some of the old skeletal structures.
"We discovered this mechanism [in living corals] because we were always looking at the same 243 coral colonies, even at those that were already dead," said Kersting. "This eventually allowed us to observe that some of the colonies we considered completely dead were actually showing living parts many years after the mortality event that killed them. Once the polyp has survived and spreads over dead areas through budding, nothing gets in its way, even outcompeting algae that settled on the dead colony areas."
Just as fossil corals gave the researchers clues that helped them understand the mechanism they were observing in living corals, Kersting noted that the finding could shed light on the evolution of survival strategies in ancient colonies. For example, on a few occasions during this study the team found up to three new calices inside the original, potentially explaining the origin and causes of similar skeletal structures described in fossil corals.
Since. C. caespitosa is the first species in which rejuvenescence has been observed, it is difficult to draw conclusions about the survival strategies of other modern reef-builder coral species, such as those in the tropics. However, since this mechanism has been found in a modern species after first being described in entirely different extinct corals, Kersting said rejuvenescence or something similar could exist in other living varieties.
"Now that we have this information, we have to look for these kinds of strategies in other species and assess their ecological importance," said Kersting. "It is also very important to continue working on our long-term monitoring transect and assess how this process evolves through time and in relation to the oncoming marine heat waves, which, unfortunately, will come, and probably with increased frequency."
The study's results could also be used to further boost recovering C. caespitosa colonies by informing better management practices that protect seemingly dead colonies, recognizing they may yet recover if left alone.
However, Kersting cautioned that even this unforeseen coral superpower is no match for a future world in which humans fail to mitigate climate change.
"These kinds of survival and recovery strategies are showing us that coral recoveries are still possible, but only in a scenario of decreasing greenhouse gases emissions and slowed warming rates," said Kersting. "We insist that if warming rates and the frequency of marine heat waves keep on increasing, warming-related mortalities are going to outpace the recovery capacity of these corals."