Study Documents Hollow Coral Skeletons Cemented in Place by Encrusting Algae on Moorea

A new study on Moorea documented hollowed-out dead coral branches held in place by encrusting red algae and internal microorganisms, a process not previously described in scientific literature. D. student and the study’s lead author, made the initial observation during a 2020 survey dive when a dead coral branch she grasped snapped off and proved entirely hollow inside.

Peter Edmunds, a biologist and professor at California State University, Northridge who has studied corals for more than 40 years, found a similar structure on his own dive and had never encountered the condition before. Samples were sent to Bruce Fouke, a geologist and professor at the University of Illinois, who examined them under a microscope capable of imaging to one billionth of a meter.

Fouke determined that mollusks, fungi, and bacteria had mined the calcium skeleton from within while encrusting red algae on the exterior accreted additional calcium carbonate, forming a natural cement.

The algae also produce enzymes, lipids, and proteins that render the surface inhospitable for larval coral settlement. Inside Climate News reported that the hollowing-and-scaffolding process has prolonged the interval between coral mortality and rubble formation, thereby extending the period during which recovery remains blocked.

Moorea lies in French Polynesia, 2,700 miles south of Hawaii, where volcanic ridgelines extend underwater and once supported a resilient reef system.

The Moorea Coral Reef Long-Term Ecological Research Program has tracked reef recovery following a 2019 bleaching event that killed substantial coral cover. Hannah Stewart, head of science for the nonprofit Coral Gardeners, said during a recent dive she counted three live corals in 45 minutes.

Stewart also noted that prior recovery on Moorea had depended on cyclones clearing dead skeletons and exposing fresh substrate for larval settlement.

Bill Precht, director of coastal and marine science programs at Columbia University’s School of Professional Studies, said Moorea’s recovery rate has been comparatively rapid because of upstream and downstream larval sources. Edmunds stated that present-day reefs function under warmer seawater and more frequent storms, so older recovery patterns may no longer apply.

Scafidi observed that the reef appeared in better condition immediately after the 2019 bleaching event than it does at present.

The study noted anecdotal reports of similar hollowing on nearby islands, though the phenomenon has not yet been systematically documented elsewhere. The research was supported by the Pulitzer Center. The team intends to continue monitoring the site and has encouraged other researchers to examine local reefs for comparable internal dissolution and external cementation processes.