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.
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.
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.
“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.
The images they collected revealed a microcosm of organisms at work on the coral skeletons.
“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.
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.
The NSF LTER program and the Barbara and Edward Weil Foundation supported this research.
Bruce Fouke is also a professor in the Cancer Center at Illinois and the School of Integrative Biology and the director of the Roy J. Carver Biotechnology Center at the U. of I.
Editor’s note:
This news release is the product of human, not artificial, intelligence.
To reach Peter Edmunds, email peter.edmunds@csun.edu.
To reach Kathryn Scafidi, email kathryn.scafidi@postgrad.otago.ac.nz.
To reach Bruce Fouke, email fouke@illinois.edu.
The paper “Remnant hollowed out dead coral skeleton branches defer coral community recovery” is available online.
DOI: 10.1371/journal.pone.0339527
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