Bleached Corals Thrive Within Days When Touching Healthy Neighbour

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Researchers report that bleached fragments of a stony coral recovered after touching healthy fragments from the same genetic individual. The laboratory results suggest coral contact can transfer symbiotic algae, but the method has not been shown to work at reef scale.

Bleached fragments of the coral Pocillopora damicornis recovered in laboratory tanks after touching healthy fragments from the same genetic individual, researchers report in a study published October 14 in Science Advances. The experiment found that contact allowed symbiotic algae to move into the bleached coral, offering a possible avenue for restoration research but not yet a demonstrated reef-scale treatment.

The work began after marine scientist Stephen Palumbi noticed in early 2025 that colour appeared to be moving from a healthy coral fragment into a bleached fragment where the two were touching. Study co-author Danny Collins then tested whether the coral’s symbiotic algae could spread through contact.

Collins prepared 10 pairs of fragments from the same coral genet, with one healthy and one bleached fragment in each pair. Five pairs were placed in contact; the other five were kept a few millimetres apart. In the touching group, bleached pieces recovered within eight weeks. The researchers reported that the algal symbiont population was doubling every four days by the second week.

After a bleached branch had touched its healthy counterpart for one week, it could continue recovering after the fragments were separated. The separated pairs did not recover and died after 20 weeks. Using high-powered microscopy, the researchers observed a tissue bridge between touching fragments, which they say enabled the transfer. Fragments from different species, or different animals of the same species, did not fuse in the tests.

A Possible Tool for Coral Recovery

The result points to a biological process that might help restore some bleached corals: a healthy fragment could supply symbiotic algae to a compatible bleached fragment through a tissue connection. Those algae provide corals with much of their nutrition, so their return is central to recovery from bleaching.

The findings are a laboratory result, not evidence that the approach can reverse bleaching across reefs. The experiment involved a small number of paired fragments from one coral species and required fragments from the same genet to fuse. Whether researchers can use the process safely and effectively in larger restoration efforts remains untested. Fiorenza Micheli, a marine ecologist at Stanford University and study co-author, said the finding could inspire solutions that may eventually be scalable.

How Bleaching Removes Coral Algae

Corals live in partnership with microscopic algae known as Symbiodiniaceae. The algae live in coral tissues and provide much of the animals’ nutrients through photosynthesis. When corals face stress, including rising sea temperatures, they can expel the algae. The resulting loss of colour is called bleaching; without their usual source of nutrients, corals may starve and die.

Recovery can happen when conditions improve and corals regain symbiotic algae, but this study examined a different route: direct transfer between touching fragments. The researchers’ observation that the fragments fused only when they came from the same genet indicates that compatibility may limit how widely this method can be applied.

“I’d never seen that before.”

— Stephen Palumbi, marine scientist and study co-author

Limits Beyond the Laboratory

The study does not establish whether the same recovery can be achieved in natural reef conditions, across other coral species, or at a scale large enough to affect reef decline. The reported test used 10 pairs of fragments from one genet of Pocillopora damicornis; the source material does not describe trials across multiple genets or field deployments.

It also remains unclear how often compatible fragments would be available in restoration settings, how long recovered corals would remain healthy, and whether the tissue-fusion process can be managed without unintended effects. The findings do not address the underlying heat stress that causes bleaching. Researchers have not shown that contact-based recovery can prevent further bleaching when temperatures remain high.

Testing Contact-Based Restoration

The immediate next step is further research into how reliably the process works across coral genets, species and environmental conditions. Field testing would be needed to determine whether fragments can fuse and transfer symbionts outside controlled laboratory tanks, and whether recovered corals persist after transplantation or renewed heat stress.

No reef deployment or timetable for practical use is reported in the source material. For now, the study establishes a laboratory finding: direct contact between fragments from the same genet was associated with recovery, while separated fragments in the comparison group died. Whether that mechanism can contribute to real-world coral restoration remains an open question.

Key Questions

What did the researchers find?

In laboratory tests, bleached Pocillopora damicornis fragments recovered after touching healthy fragments from the same genet. Microscopy showed a tissue bridge between the touching pieces, which researchers linked to the transfer of symbiotic algae.

How quickly did the coral recover?

The touching fragments recovered within eight weeks. By the second week, the researchers reported that the symbiont population was doubling every four days. A bleached branch that had touched a healthy one for a week could continue recovering after separation.

Did separated coral fragments recover?

No. In the experiment, the five pairs kept a few millimetres apart did not form the tissue bridge and the bleached fragments died after 20 weeks.

Can this method restore coral reefs?

That has not been demonstrated. The work was conducted in laboratory tanks with a small set of fragments from one coral genet. Field trials and tests across more corals would be needed to establish whether the approach can be used in reef restoration.

Why does bleaching harm coral?

Bleaching occurs when stressed corals expel symbiotic algae that provide much of their nutrition. Without those algae, corals can be left short of nutrients and may die, especially if the stress continues.

Source: hn