For the first time, scientists have observed bleached corals on the brink of death fully recover through brief contact with healthy clones.
Ghostly white corals returned to full health in a laboratory setting within seven weeks, the fastest documented recovery to date, according to a study published Oct. 14 in Science Advances . The findings could inspire the development of new methods to rapidly restore corals, which, by some estimates , could disappear within a few decades as climate change drives more frequent and intense marine heat waves .
When waters warm, corals expel colorful, beneficial algae that live in their tissues and provide the energy that corals need to grow. The loss of their algae partners leaves corals pale, starving, and vulnerable to the invasion of harmful algae. If corals survive these bleaching events, it can take months to years for them to fully recover.
"While bleached corals may be dying, they are not dead yet," said lead author Danny Collins , an oceans PhD student at the Stanford Doerr School of Sustainability . "The question we wanted to answer is, once a coral has bleached, what can we do?"
Jump-starting coral recovery
All corals are animals. Across most species, individual corals multiply by producing clones that grow together to form vast, reef-building colonies of the same genetic makeup.
In 2025, study coauthor Steve Palumbi , the Jane and Marshall Steel, Jr. Professor in Marine Sciences, observed something curious about coral biology in his research lab. Two coral fragments – one bleached, one healthy – sampled from the same colony had tipped over in a tank and were now touching. Where they made contact, color returned to the bleached fragment, signaling the return of beneficial algae.
Around the time of this observation, Collins was considering the origins of coral disease, particularly the transmission of bacteria lurking on plastic debris that settles onto corals.
"Harmful bacteria enter corals through physical contact," said Collins, who is also a Knight-Hennessy Scholar . "We were curious about what would happen if we used contact to reintroduce something good – the beneficial algae from healthy corals."
Collins set up a simple experiment. He assembled 10 pairs of coral fragments – one bleached, one healthy – from the same animal. He arranged five pairs close enough to touch, and left gaps of just a few millimeters between the remaining pairs. The bleached fragments in touching pairs fully recovered within seven weeks, while those without contact died.
"Genetically identical corals recognize one another when they come into contact," said Palumbi, who is a professor of oceans in the Doerr School of Sustainability and of biology in the School of Humanities and Sciences . "Their tissues fuse if they are from the same clone. We see something similar in humans. Organ transplants are often successful if the donor is your identical twin."
Collins and team used a powerful microscope to discover that microscopic channels in coral tissues align shortly after contact, creating a pathway for transport between corals. Beneficial algae and the nutrients they produce then flow through these channels. The team observed that beneficial algae moved from healthy to bleached fragments within three days of contact.
The team also tested how different contact durations influenced recovery rates. They separated coral pairs at varying time intervals and found that after one week of contact, and in some cases after just five days, bleached fragments received enough beneficial algae to complete recovery on their own.
After separation, recovery took just seven weeks, matching the recovery time of coral pairs left in continuous contact. This led the team to realize that algae replicating within the bleached fragment, rather than traveling from the healthy fragment, primarily drove continued recovery.
"Once beneficial algae enter bleached corals, they don't just spread out," said Palumbi. "They flood in and start dividing at the fastest rates seen in corals."
During the second week of contact with healthy corals, algae within bleached fragments doubled every four days, roughly ten times faster than the rate observed in wild corals.
Renewed hope for coral reefs
For several decades, researchers have raced to develop tools and approaches that can protect corals from bleaching, disease outbreaks, and increasingly acidic waters, among other threats. Solutions include selectively breeding heat-tolerant corals , cultivating coral nurseries to restore degraded reefs , and even transplanting coral microbiomes.
"Bleached corals are often considered a lost cause because recovery after a heatwave is very slow, if it happens at all," said Fiorenza Micheli , the David and Lucile Packard Professor in Marine Science and a faculty advisor of Collins. "We hope this new research will inspire solutions that keep corals healthy long enough to enable full recovery after heat waves through slower processes such as growth, reproduction, and recruitment of new colonies."
The study team cautions that while the healing potential of physical contact is promising, many questions still remain. The approach requires a genetically identical, healthy copy of every bleached coral, which is not feasible at the scale needed to restore reefs.
In the coming months, the researchers plan to explore alternative ways of delivering beneficial algae to bleached corals that don't require continuous contact. They anticipate that retrieving algae from healthy corals long before heat waves hit, and then delivering them in large batches to bleached corals, may be one way to accelerate recovery.
"These new insights into how corals can naturally recover give us hope for the future," said Micheli, who also co-directs the Stanford Center for Ocean Solutions . "They open the door to what's possible and may lead to the breakthrough we need to accelerate reef restoration."
Micheli is also a professor of oceans and (by courtesy) of biology. She and Palumbi are senior fellows at the Woods Institute for the Environment, part of the Stanford Doerr School of Sustainability.
Stanford coauthors not mentioned above include Jana Sipkova , Nathaniel Mollica , Tom Rolander , Courtney Klepac , Gabriel Calistro , and Vanessa Barone . Mollica is a research scientist and Calistro is a research assistant in the Oceans Department. Rolander is a research engineer in the Fabrication Lab at the Harold A. Miller Library at Hopkins Marine Station , part of the Doerr School of Sustainability.
Sipkova is a postdoctoral scholar, Klepac is a research scientist, and Barone is an assistant professor in the School of Humanities and Sciences' Biology Department. Barone is also a member of Bio-X and the Maternal & Child Health Research Institute.
Coral image analyses were provided by the Molecular and Cellular Biodiversity Imaging Center , which the Biology Department operates at Hopkins Marine Station.
Funding was provided by the National Science Foundation and Stanford University, including seed funds from the Big Ideas for Oceans grant program hosted by the Oceans Department and the Woods Institute for the Environment in the Stanford Doerr School of Sustainability.
