Stronger Together: Isograft Fusion Accelerates Recovery and May Facilitate Heat-Evolved Symbiont Transmission in Bleached Adult Corals.

Experimentally evolved, heat-tolerant algal symbionts (heat-evolved; HE) offer a promising means of enhancing coral holobiont thermotolerance under warming oceans. However, translating their benefits into restoration practices requires scalable delivery methods. Coral tissue fusion may provide one such pathway by facilitating HE symbiont transfer to wild corals; however, its feasibility remains largely untested. As an initial test, we paired adult isografts of Galaxea fascicularis and Psammocora columna hosting HE Cladocopium proliferum (SS8) with chemically bleached, SS8-naive recipients. Fusion was first observed after 3 days in G. fascicularis and 9 days in P. columna. In both species, fusion was followed by increased pigmentation and photochemical efficiency at the recipient's fusion interface relative to distal tissue and unfused controls. After ~50 days, SS8 was detected at low levels (< 3.5%) in 15/19 fused G. fascicularis recipients, although detection was also common among unfused horizontal-transmission controls maintained in the same water column (13/18). These findings provide the first empirical evidence that conspecific coral tissue fusion is associated with rapid localised physiological recovery and can coincide with HE symbiont acquisition, but also highlight the need to distinguish tissue-mediated transfer from background horizontal transmission. Fusion may therefore represent a complementary pathway for symbiont delivery in assisted-evolution frameworks.