Most stony corals are obligate symbionts that are dependent on nutrients provided by the photosynthetic activity of dinoflagellates residing within specialized cells1. Disruption of this symbiotic consortium leads to coral bleaching and, ultimately, mortality2. However, a few coral species exhibit facultative symbiosis, allowing them to survive extended periods of bleaching3,4. Despite this resilience, the underlying biological mechanisms remain poorly understood. Here we investigate the genomic and cellular basis of facultative symbiosis in Oculina patagonica, a thermotolerant Mediterranean coral5,6. We sequenced and annotated a chromosome-scale genome of O. patagonica and built cell atlases for this species and two obligate symbiotic corals. Comparative genomic analysis revealed karyotypic and syntenic conservation across all scleractinians, with species-specific gene expansions primarily driven by tandem duplications. Single-cell transcriptomic profiling of symbiotic and naturally aposymbiotic wild specimens identified an increase in phagocytic immune cells and a metabolic shift in gastrodermal gene expression from growth-related functions to quiescent, epithelial-like states. Cross-species comparison of host cells uncovered Oculina-specific metabolic and signalling adaptations indicative of an opportunistic, dual-feeding strategy that decouples survival from symbiotic state.
