Coral reefs are struggling as ocean temperatures rise and bleaching events follow. When corals die, the consequences can spread across entire ecosystems that rely on reefs, but a laboratory accident has revealed a striking possibility: some bleached corals may recover with direct help from healthy neighbors.

The discovery emerged while scientists were conducting a different experiment involving Pocillopora damicornis. Two samples had been left touching in the laboratory, with one coral bleached and the other healthy.

Instead of remaining separate, the healthy and bleached corals appeared to fuse. That connection created a route through which algae living in the healthy coral could move into the bleached sample.

Study author Danny Collins of Stanford University said the speed of the process caught the researchers off guard. “We were most surprised by the speed of the recovery. We hypothesized that some transfer of the beneficial algae may occur, but never thought it would occur that quickly,” Collins told IFLScience.

After just one week of contact, the bleached coral had fully recovered and remained healthy even after being separated from its donor. In some cases, tissues from both corals ripped during separation, suggesting that tissue fusion helped facilitate the transfer.

The results extended beyond a single pair. “All bleached coral in direct contact with healthy coral (n = 5) achieved complete recovery within 8 weeks,” the authors explained in their paper.

Some recovered so thoroughly that researchers documented new polyp growth. That observation meant the formerly bleached corals were actively producing new tissue rather than merely regaining their previous appearance.

The team used microscopy, tracked the movement of algae and separated corals at different stages of recovery to identify fusion as the transfer mechanism. Collins described the connection as a tissue bridge that allowed healthy coral to deliver algae directly to bleached coral.

“After 2-3 days of contact, the tissues of the healthy and bleached corals fused together, forming a tissue bridge that acted as a nutrient highway, enabling the healthy coral to transfer its algae to the bleached coral, ultimately driving its recovery,” Collins said.

To determine whether physical contact was essential, the researchers placed bleached fragments 5 to 7 millimeters, or 0.19 to 0.27 inches, away from healthy fragments. Those corals remained bleached and even worsened despite being close to healthy samples.

Bleached corals can retain remnant algae, but Collins explained that heat stress and damage cause those algae to replicate slowly, limiting coral recovery. Algae transferred from healthy corals were healthy and undamaged, allowing them to multiply much faster.

The researchers hypothesize that bleached corals may regulate the growth of newly introduced algae less effectively. That could allow the algae to multiply rapidly and propel the recovery process.

Successful recovery also depended on the coral pair coming from the same genet, meaning a genetically unique colony with shared ancestry. When corals from different genets were paired, they became antagonistic and grew stinging nematocyst filaments.

Different genet pairs showed some algae uptake, but not enough to support complete recovery. Collins said the requirement for contact between corals of the same clone currently limits the finding’s immediate application on reefs.

In same genet pairs, researchers observed different algae flow patterns depending on where the healthy and bleached corals were connected. Corals fused at the top showed algae moving downward, while those joined at the bottom showed algae moving upward.

One pair fused at both the top and bottom. In that case, algae moved simultaneously and at the same rate through both openings.

The team is now exploring how the laboratory findings might be translated into interventions for bleached reefs in the wild. Researchers are investigating whether algae could be transferred without requiring contact with a healthy coral.

Beneficial algae can be extracted from healthy corals and cultured, and the researchers are examining bioengineering approaches for returning them to bleached corals. Collins said the team hopes eventually to explore solutions that could be deployed in the field.

Considerable work remains before the method could be used to treat bleached and damaged reefs in wild environments. Still, the findings show that bleached coral is not necessarily beyond recovery once its symbionts are restored.

“This is a recovery story,” Collins said. The study was published in Scientific Advances.