Structural Stagnation in French Polynesia: Encrusting Algae and Internal Mining Hinder Recovery of Moorea’s Bleached Coral Reefs

New research shows that dead coral in Moorea is being "hollowed out" but stabilized by algae, preventing the natural clearance needed for reef recovery.

By: AXL Media

Published: Mar 13, 2026, 6:44 AM EDT

Source: Information for this report was sourced from University of Illinois at Urbana-Champaign, News Bureau

The 2019 Bleaching Event and the Stalled Recovery Cycle

The island of Moorea was once a primary example of coral resilience, famously rebounding from a massive crown-of-thorns starfish infestation in the early 2000s and a 2010 cyclone. However, a marine heat wave in April 2019 shifted this trajectory, reducing live coral cover from 75% to less than 17% in a single year. Typically, dead coral skeletons are dislodged by tropical storms, clearing the "geological rock" to allow for a new generation of coral. According to new research published in PLOS One, this essential cycle of destruction and repair has stalled due to an unexpected biological interaction that is keeping dead, "zombie" reefs standing long after they have lost their living tissue.

An Unprecedented Discovery of Hollowed Skeletons

During a 2022 data collection dive for the Long-Term Ecological Research Network, Ph.D. student Kathryn Scafidi made a startling discovery: standing coral branches that were entirely hollow. Professor Peter Edmunds, a veteran of four decades in reef biology, noted that while coral is naturally porous, standing branches should remain solid stone-like structures. High-resolution microscopy at the University of Illinois revealed that the interiors of these dead branches had been "mined" by tiny mussels and fungi seeking shelter. This internal erosion should have made the reef fragile and easy for waves to clear, yet the skeletons remained firmly in place.

External Stabilization by Encrusting Algae

The mystery of why these hollowed structures did not break apart was solved by identifying the role of Lobophora variegata, a type of brown alga. This organism has formed a dense, encrusting veneer over the dead coral, effectively acting as a biological glue. This external stabilization is so potent that even a significant tropical storm in 2024 failed to dislodge the dead skeletons. Scafidi explains that this "encasing" prevents the clearing of the seabed, leaving no physical space for juvenile corals to attach and begin rebuilding the reef, a phenomenon that has not been documented in previous recovery phases.