From bleaching to restoration: mechanisms of mutualism recovery in Exaiptasia and implications for coral symbiosis

Coral bleaching, driven by ocean warming and other anthropogenic stressors, is the breakdown of the mutualism between reef-building corals and their algal symbionts. While most corals do not die immediately after bleaching, they often remain refractory to symbiont reinfection, hindering reef recovery. In contrast, other cnidarians, such as the sea anemone Exaiptasia diaphana and the upside-down jellyfish Cassiopea, can successfully re-establish symbiosis after bleaching. Exaiptasia is thus emerging as a comparative biology model for dissecting the cellular and molecular processes underlying symbiosis establishment, maintenance, and breakdown. Comparative analysis of corals with Exaiptasia reveals traits that support recolonization, including broad symbiont compatibility, sustained uptake competence, structural plasticity of the symbiosome, and physiological resilience in the aposymbiotic state. Environmental conditions such as nutrient stoichiometry, light spectra, and thermal history further modulate recolonization dynamics in controlled systems. These findings identify candidate mechanisms and generate testable hypotheses relevant for addressing recolonization bottlenecks in corals. We evaluate the extent to which these mechanisms may be relevant to reef-building corals, emphasizing both conserved features of cnidarian symbiosis and key biological differences that constrain direct extrapolation. While adult corals can in some cases reacquire symbionts following bleaching, this process is often context-dependent and may be limited in stability and ecological relevance.