Rapid coral bleaching recovery via symbiont transfer from healthy corals.

Coral bleaching, the temperature-induced loss of dinoflagellate symbionts, is a major driver of global coral reef decline. Although corals can regain symbionts through repopulation of residual symbionts or via environmental acquisition, these processes are often too slow to prevent coral mortality. Mechanisms that enable rapid and effective symbiont reintroduction leading to full recovery remain largely unknown. Here, we demonstrate coral bleaching recovery through contact-mediated transfer of symbionts from healthy corals using small coral fragments. We show that symbiont transfer is rapid, occurring over days to weeks, and highly efficient, with only brief physical contact with healthy corals required. A single week of contact, and in some cases as few as 3 days, provides sufficient symbionts to initiate sustained and full self-recovery. By 1 week, symbiont population doubling time is approximately 4 days, up to an order of magnitude faster than most recovery times for corals in the field. We identify tissue fusion as the mechanism enabling directional symbiont transfer from healthy to bleached coral tissue. As a result, contact-dependent recovery is restricted to corals of the same genet, whereas heterologous genet pairings trigger antagonistic interactions. However, symbiont transfer via polyp uptake occurs immediately upon contact in both same-genet and mixed-genet pairings, revealing an additional, slower pathway for symbiont transfer that is insufficient on its own to support colony recovery. Collectively, these findings reveal a previously unrecognized natural process for efficiently reintroducing symbionts to bleached corals, providing a foundation for research into potential recovery interventions.