Still standing but mostly dead: Recovery of dying coral reef in Moorea stalls

Dead branches of Pocillopora coral on the outer reef of Moorea were killed by bleaching in 2019. The dead branches are coated in algae and the broken ends expose the hollow interior that is described in a new study.
Photo Credit: Kathryn Scafidi

Scientific Frontline: "At a Glance" Summary: Coral Reef Recovery Stalls in Moorea

• Main Discovery: Dead coral branches in Moorea are being hollowed out internally by marine organisms like mussels and fungi, while their exteriors are simultaneously fortified by encrusting algae, creating durable but dead structures that prevent new coral from growing.
• Methodology: Researchers collected long-term ecological field data via scuba surveys and utilized high-resolution microscopy to analyze the structural integrity, porosity, and biological composition of the intact but hollowed-out coral skeletons.
• Key Data: A 2019 marine heat wave triggered a severe bleaching event that reduced live coral coverage on the affected Moorea reef from approximately 75% to less than 17% within a single year.
• Significance: The unprecedented structural stabilization of dead coral by the alga Lobophora variegata disrupts the natural cycle of reef regeneration, as the enduring skeletons fail to break away and thereby occupy the essential physical space required for juvenile corals to settle and recolonize.
• Future Application: These findings will refine predictive ecological models regarding coral reef degradation and inform targeted marine intervention strategies to facilitate reef recovery in environments facing chronic warming and acute marine heat waves.
• Branch of Science: Marine Biology, Earth Science, and Environmental Ecology.
• Additional Detail: The structural integrity provided by the encrusting algae allowed the dead coral skeletons to successfully withstand a 2024 tropical storm that would have typically cleared the debris to make room for new growth.

Living Pocillopora coral at 2 meters depth on the back reef of Moorea.
Photo Credit: Peter Edmunds

In April 2019, a marine heat wave struck a coral reef on the island of Moorea in French Polynesia, killing much of the coral and the beneficial algae that colonized it. This “bleaching” event reduced live coral populations on the reef from about 75% beforehand to less than 17% a year later and led to a series of unexpected changes that have thwarted the reef’s recovery.

A long-term study of the area is challenging scientists’ understanding of the cycles of destruction and repair that can occur on a coral reef. The new findings are reported in the journal PLOS One.

Thousands of organisms contribute to the life of a reef, and each plays a specific role, said Peter Edmunds, a professor of biology at California State University, Northridge, who co-led the new study with former CSUN research technician Kathryn Scafidi and University of Illinois Urbana-Champaign earth science and environmental change professor Bruce Fouke. Scafidi is first author of the paper and now a Ph.D. student at the University of Otago in Dunedin, New Zealand.

Peter Edmunds prepares his camera gear to record changes on the coral reefs of Moorea in 2023.
Photo Credit: A. Dahl

Algae are one of the dominant life forms on a reef, Edmunds said.

“Many people have argued that a coral reef shouldn’t really be called a coral reef,” he said. “It would be better to call it an algal reef upon which corals are living because algae are now ubiquitous.”

The algae are as diverse as they are abundant. Some are symbiotic single-celled organisms that live in coral tissues and photosynthesize to produce oxygen, which the coral need to live. Others are multicellular and large, including a type of brown algae that tightly adheres to the dead corals’ hard surfaces.

“We found that this alga in its encrusting form has coated many of the dead corals after the bleaching event in Moorea and appears to be impeding the reef’s recovery,” Scafidi said.

Study first author Kathryn Scafidi works at 10 meters depth on the reefs of Moorea to measure dead Pocillopora corals. Here she is applying a “crush assay” to dead branches to determine whether they are hollow.
Photo Credit: D. Becker

An unexpected discovery led to this finding. In 2022, Scafidi and Edmunds were scuba diving to collect data from the Moorea reef as part of the Long-Term Ecological Research Network, a National Science Foundation initiative that collects ecological data over time from 27 sites across the United States, French Polynesia and Antarctica. Scafidi was recording the status of corals and Edmunds was photographing them.

“I happened to place my hand down on a dead coral skeleton, just to brace myself for a little bit of the surge and one of the branches broke off and it was completely hollow inside,” Scafidi said. She showed Edmunds.  

Coral is not normally hollow, said Fouke, who was recruited to analyze the hollow branches using advanced microscopy techniques.

U. of I. professor Bruce Fouke’s team used microscopy to analyze the physical characteristics of the hollow coral branches.
Photo Credit: Fred Zwicky

“Coral reef systems are a living, breathing form of geology,” he said. “As soon as the coral is formed, it is already more than 90% skeleton, and you have a ready-made rock.” In a healthy reef, this skeleton forms solid coral branches. “Overlying this is a thin veneer of living tissue that contains tiny symbiotic algae. The algae give the coral most of its color and the skeleton is white.”

Coral branches are porous, allowing sea water to percolate through them, but they are not hollow, Edmunds said. In four decades studying coral reefs around the world, he said, he’s never seen or read about standing coral branches that had hollowed out. Normally, the dead coral breaks up and washes away in a storm, making room for juvenile corals to recolonize the reef.

Study co-authors Mayandi Sivaguru, an expert microscopist at the U. of I. Roy J. Carver Biotechnology Center, and Kyle Fouke, an NSF postdoctoral fellow at the University of Wisconsin Madison, used high-resolution microscopy to view the coral specimens.

Three views of the coral include, top left, a fish-eye view of the algae-encrusted coral; top right, a microscopic view of one wall of the hollow coral; and, bottom, a high-resolution micrograph of a section of the same coral. This analysis revealed that a host of tiny organisms were simultaneously stabilizing the coral’s exterior surfaces and hollowing out its interior.
Image Credits: Micrographs by Mayandi Sivaguru and Kyle Fouke. Top left photo by Peter Edmunds.

The images they collected revealed a microcosm of organisms at work on the coral skeletons.

Three views of the coral include, top left, a fish-eye view of the algae-encrusted coral; top right, a microscopic view of one wall of the hollow coral; and, bottom, a high-resolution micrograph of a section of the same coral. This analysis revealed that a host of tiny organisms were simultaneously stabilizing the coral’s exterior surfaces and hollowing out its interior. Micrographs by Mayandi Sivaguru and Kyle Fouke. Top left photo by Peter Edmunds. 

“The exterior surfaces were encased in algae, trapped sediments and other encrusting organisms such as worms and sponges,” Bruce Fouke said. “The interiors were riddled with holes of varying sizes, each the work of tiny mussels and fungi that appear to have mined the coral interior.”

These organisms are most likely looking for shelter, Edmunds said.

Scafidi also found that the encrusted dead corals appeared to be more resistant to breakage.

The findings suggest that one algal species, Lobophora variegata, is stabilizing the dead corals while the interiors of the coral branches are being hollowed out, Scafidi said. Because of this, a tropical storm that came through in 2024 did not dislodge the dead coral skeletons as previous storms had.

Juvenile corals need space to establish themselves and build a new reef, Scafidi said.

“Normally, these reefs in the South Pacific have rebounded after destructive events, as they did after a horde of hungry crown-of-thorns sea stars came through the Moorea reefs in the early 2000s, killing hectares of the standing coral,” she said. “In 2010, a large cyclone blew through and cleared out most of that dead coral.

“The coral cover was probably less than 5% at that time, and then by 2019, just before the bleaching, it was at about 75% live coral cover,” she said.

Living Pocillopora coral at 2 meters depth on the back reef of Moorea.
Photo Credit: Peter Edmunds

Edmunds said the findings reflect the many factors that affect the life, death and potential recovery of a reef. Marine temperatures are rising incrementally each year, a change that is almost imperceptible, but which adds up over time.

“On top of that, you have these signals that now are described as marine heat waves that are epic acute disturbances on top of the chronic disturbance,” he said.

Heat waves are not a new phenomenon, but they may have more, or different impacts as the temperature rises, causing more disturbance to the balance of organisms that are successful on the reef, Scafidi said. This may be making a reef more attractive to more competitive species.

“This gives us a fundamentally new perspective on the rate at which corals are breaking down in a human-perturbed world, and that is critical in understanding how or when or whether coral reefs will recover,” Edmunds said.

Funding: The NSF LTER program and the Barbara and Edward Weil Foundation supported this research.

Published in journal: PLOS one

Title: Remnant hollowed out dead coral skeleton branches defer coral community recovery

Authors: Kathryn C. Scafidi, Kyle W. Fouke, Mayandi Sivaguru, Bruce W. Fouke, and Peter J. Edmunds

Source/Credit: University of Illinois Urbana-Champaign | Diana Yates

Reference Number: mb031126_01

Privacy Policy | Terms of Service | Contact Us