Coral reefs are feeling the effects of climate change Images & Stories / Alamy Stock Photo

Reef-building corals can make unexpected recoveries from climate change-induced destruction. It turns out that some corals only look dead when exposed to unusually warm water. Instead, the coral’s polyps shrink and retreat into their hard skeleton, making the reef appear dead, before recolonising the skeleton when conditions are better. It is a survival strategy never seen before in today’s corals – but it may not help the corals as the climate continues to change.

Corals have been hard hit by warming waters. Reefs worldwide, including the Great Barrier Reef, are edging towards collapse. The slow-growing endangered species Cladocora caespitosa is particularly vulnerable to destruction with little indication so far as to whether it can recover.

By monitoring colonies of C. caespitosa in the Mediterranean Sea for 16 years, Diego Kersting and Cristina Linares at the University of Barcelona, Spain, have observed that recovery is possible. They discovered that seemingly dead corals can in fact regrow in the wake of heat damage caused by climate change. Some made an almost full recovery.


When the polyps that make up a C. caespitosa colony are hit by warm weather, Kersting and Linares found that they shrink and recede deep within the coral skeleton. To the eye the hard coral looks devoid of life. But given time these tiny polyps – the characteristic “tentacles” on coral – can regrow.

Ancient strategy

We already knew that ancient corals could do this, as their fossils contain the fossilised remains of tiny skeletal structures that formed when the polyps regrew. Until now, however, it wasn’t clear whether today’s corals could. “We can see why and when this strategy is put in place and how the recovery process evolves through time,” says Kersting.

Spotting one of these “recovered” corals is difficult because they look healthy, which might be why biologists haven’t recorded this survival strategy in today’s corals until now. Kersting and Linares noticed the behaviour only because they monitored corals over several years and witnessed “dead” ones returning to life.

This and other survival strategies used by different corals still present only a narrow window of opportunity for rebuilding the world’s reefs. Kersting says that it won’t be enough to compete with climate change if warming continues at its current rate and calls for similar long-term monitoring for other susceptible spots.

This particular strategy will be “highly ineffective” under current environmental change scenarios, says Gergely Torda at James Cook University, Australia. “Adaptation and acclimatisation is the only chance corals have – in tandem with us doing our part and curbing greenhouse gas emissions,” he says.

Journal reference: Science Advances, DOI: 10.1126/sciadv.aax2950