A Resilient Reef Shows New Signs Of Trouble
Moorea, a volcanic island in French Polynesia, has long been known among coral scientists as a place of remarkable reef resilience.
Its ancient lava ridgelines extend from steep green mountains into the sea, creating rocky underwater structures where coral reefs have historically flourished. But new research suggests that parts of Moorea’s reef may now be trapped in a troubling state between life and death.
Dead Corals Are Not Breaking Down
A new study has documented a phenomenon not previously described in scientific literature: dead corals are being hollowed out from the inside while remaining structurally held in place by encrusting algae.
Normally, when corals die, storms help clear the dead skeletons from the seabed. That process creates open space where young corals can settle and grow. In Moorea, however, dead coral structures are staying in place instead of breaking down.
A Reef Stuck Between Death And Recovery
The problem is that the dead coral skeletons are not becoming rubble and are not being removed by storms.
Because no clean substrate is exposed, new corals have fewer places to attach and survive. This disrupts the natural recovery cycle that has helped Moorea’s reefs rebound after past bleaching events and cyclones.
A Bleak Underwater Picture
Divers now describe large stretches of grey dead coral rubble, with only occasional living corals still holding on.
Hannah Stewart, head of science for the local nonprofit Coral Gardeners, said she once saw only three living corals during a 45-minute dive. For researchers used to Moorea’s reputation as a resilient reef system, the scene is alarming.
Climate Stress Weakens Coral Recovery
Corals are already under growing pressure from marine heatwaves and bleaching events. When water becomes too warm, corals expel the colorful algae living in their tissues, causing them to turn white and lose a critical energy source.
Repeated bleaching events weaken corals and reduce their ability to reproduce. That makes recovery harder, especially when dead skeletons are not being cleared away to make space for new growth.
A Discovery Made By Accident
The hollow coral phenomenon was discovered during a survey dive in 2020 by researcher Kathryn Scafidi, then a Ph.D. student.
While bracing herself against a dead coral in strong swell, a branch snapped off. To her surprise, it was completely hollow inside. Her adviser, coral biologist Peter Edmunds, had seen something similar during his own dive, despite never encountering such a structure in more than 40 years of coral research.
Microscopic Organisms Mine The Skeleton
Samples were sent to geologist Bruce Fouke at the University of Illinois, who examined them with an advanced microscope capable of imaging at extremely small scales.
He found that the coral’s calcium skeleton had been mined from the inside by microorganisms including mollusks, fungi and bacteria. On the outside, encrusting red algae covered the skeleton and helped maintain its structure.
Algae Acts Like Natural Cement
The red algae do more than blanket the dead coral. Their presence encourages additional calcium carbonate to form from seawater, creating what Fouke described as a kind of natural cement.
That cement-like structure keeps the dead coral standing. But the algae also make the surface hostile to baby corals by producing compounds that reduce the chances of successful settlement.
The Old Rules May No Longer Apply
Moorea’s reefs have previously recovered when storms cleared dead coral and exposed new surfaces for coral larvae.
But researchers now believe the reef can become stuck before that rubble-clearing phase. Edmunds said present-day reefs are operating in a different world of warmer seawater and more frequent storms, meaning old assumptions about reef recovery may no longer hold.
Wider Implications For Pacific Islands
Fringing reefs around volcanic islands and atolls provide protection against storm surge and flooding.
Moorea may survive even if its reef declines, but low-lying atolls that depend on reefs for wave protection and sand replenishment face a far greater threat. Without healthy coral reefs, some could eventually disappear beneath rising seas.
A Warning For Reefs Elsewhere
The phenomenon has now been formally documented in Moorea, but researchers say anecdotal evidence suggests similar processes may be occurring on nearby islands and possibly beyond.
If this pattern is regional or global, understanding it could be critical for reef restoration. Scientists now hope other research groups will examine their own reefs for signs of hollowed coral skeletons held together by algae.
Recovery Becomes Harder To Predict
Scafidi said Moorea’s reef looked damaged after the 2019 bleaching event, but in hindsight it looked better then than it does today.
The findings show how climate change can disrupt even well-studied recovery patterns. For coral scientists, Moorea’s silent pause is a warning that reef decline may not always follow familiar pathways, and that restoration strategies must adapt as ocean conditions change.
