Dead Coral in Moorea Stands Hollow and Encrusted, Blocking the Reef's Recovery

University of Illinois at Urbana-Champaign

A diver braces against the surge on a reef in Moorea, French Polynesia, and a coral branch snaps off in her hand. It is completely hollow inside. Coral is not supposed to be hollow. In four decades of reef research across the world, Peter Edmunds of California State University, Northridge, had never seen anything like it.

That accidental discovery by graduate student Kathryn Scafidi in 2022 led to a study, published in PLOS One, that reveals an unexpected obstacle to coral reef recovery: dead corals that refuse to fall down.

From 75% cover to 17% in a year

In April 2019, a marine heat wave struck the reef, triggering a mass bleaching event that killed both the coral and the symbiotic algae living in their tissues. Live coral cover dropped from approximately 75% to less than 17% within a year. This was devastating, but not unprecedented. The same reef had experienced catastrophic coral loss before.

In the early 2000s, a swarm of crown-of-thorns sea stars ate through hectares of standing coral, reducing cover to below 5%. Then, in 2010, a large cyclone swept through and physically broke apart the dead skeletons, clearing the substrate. Within nine years, coral cover had rebounded to 75%. The cycle of destruction and recovery, while brutal, was understood: something kills the coral, storms clear the debris, juvenile corals settle on the newly available space, and the reef rebuilds.

After 2019, that cycle stalled.

Encrusted on the outside, mined on the inside

The research team, which included Scafidi (now at the University of Otago), Edmunds, and University of Illinois earth scientist Bruce Fouke, used high-resolution microscopy to examine the dead coral branches. What they found was a two-front assault on the skeleton's structural integrity, but not its stability.

On the outside, a species of brown algae called Lobophora variegata had tightly encrusted the dead coral surfaces, along with trapped sediments, worms, and sponges. This algal coating essentially armored the dead branches, making them more resistant to mechanical breakage.

On the inside, tiny mussels and fungi had colonized the porous coral skeleton, boring holes of varying sizes and gradually hollowing out the interior. These organisms appeared to be seeking shelter rather than food, creating a network of cavities within what had once been solid calcium carbonate.

The result: coral branches that looked intact from the outside but were structurally empty within. Standing dead coral, held upright by an algal shell around a hollowed-out skeleton.

The storm that did not clear the reef

The critical test came in 2024, when a tropical storm passed through the Moorea area. In previous recovery cycles, storms had served an essential ecological function by physically breaking and removing dead coral, exposing bare substrate where new coral larvae could settle and grow. But the encrusted, reinforced dead branches from the 2019 bleaching event largely survived the 2024 storm intact.

This is the core problem. Juvenile corals need space. They need bare, stable substrate to attach to and grow on. As long as the dead skeletons remain standing, occupying the real estate that new corals require, the reef cannot begin the regrowth process that has historically followed mass mortality events in this area.

A new dynamic in a warming ocean

The researchers suggest that rising baseline ocean temperatures may be changing the biological community that colonizes dead coral, favoring species like L. variegata that thrive in warmer conditions and providing the dead skeletons with an algal armor they did not previously receive. Marine heat waves are becoming more frequent and intense, and each one may create conditions that are slightly different from the last, potentially altering the balance between coral destruction and recovery in ways that historical patterns do not predict.

Edmunds described the situation as a fundamentally new perspective on the rate at which corals break down in a warming world. The assumption that storms will clear the way for reef recovery, an assumption built on decades of observation, may no longer hold in an ocean where heat waves are increasingly severe and the organisms that colonize dead coral are changing.

What remains unknown

The study documents a phenomenon at a single reef site in Moorea. Whether the same pattern, algal encrustation stabilizing dead coral and preventing storm-driven clearing, is occurring on other heat-damaged reefs around the world has not been investigated. The prevalence of L. variegata colonization and the rate of internal boring by mussels and fungi likely vary with location, water chemistry, and the species composition of local reefs.

The study does not quantify how long the dead skeletons might persist. Without storm removal, these structures could stand for years or decades, but the rate at which internal boring undermines their stability is not well characterized. At some point, the hollowed-out branches will presumably collapse, but the timeline is uncertain.

The researchers also cannot determine whether the reef will eventually recover through alternative pathways, for instance, if juvenile corals begin settling on the algal-encrusted surfaces of the dead branches rather than on bare substrate. This possibility was not tested.

The finding adds to a growing body of evidence that coral reef recovery after climate-driven mortality events is neither automatic nor predictable. In Moorea, a reef that bounced back from near-total destruction within a decade now appears stuck, its recovery blocked by the very skeletons of its former inhabitants.