MOOREA — A new study documented a previously unreported phenomenon in Moorea, French Polynesia, where hollowed-out dead corals are structurally maintained by encrusting algae, preventing reef recovery after bleaching events. Scientists found no prior documentation of this hollowing-and-scaffolding process in scientific literature.

During a 2020 survey dive, Kathryn Scafidi discovered a dead coral branch that was entirely hollow inside. Researchers determined that microorganisms, including mollusks, fungi, and bacteria, had mined the calcium skeleton of corals from the inside. Meanwhile, encrusting red algae cover the outside of these dead skeletons and drive the accretion of additional calcium carbonate from seawater, effectively cementing the structures in place.

This dual process prevents the natural breakdown of dead coral rubble, which typically creates space for new corals to seed and grow after disturbances like bleaching or storms. The reef studied had not recovered from the 2019 bleaching event, and divers observed dead coral rubble stretching as far as underwater visibility allowed, with only occasional living corals present. “But now this possibility for a reef to get stuck between the mortality and rubble formation phase has prolonged any sort of possibility of recovery,” Scafidi said.

Peter Edmunds, biologist and professor at California State University, Northridge, who has studied corals for over 40 years, said he had never seen anything like the hollow structures in his career. “We were a bit flummoxed.” He added, “Present-day reefs operate in a very different world of warmer seawater and more frequent storms, so there is little reason to expect the old rules to apply to new situations.”

Bruce Fouke, geologist and professor at the University of Illinois, examined coral samples under a $1.8 million microscope capable of imaging down to one billionth of a meter. He said, “And then we got to be Sherlock Holmes, right? Doing a forensic reconstruction of the crime scene.” Fouke said, “It’s like a natural cement that forms. And the problem with algae is that they are masters at biochemical warfare,” able to produce enzymes, lipids and proteins that make the surface inhospitable for baby corals.