Changes within the coral symbiosis underpin seasonal trophic plasticity in reef corals

Scleractinian corals are mixotrophic organisms that use both autotrophic and heterotrophic pathways to fulfill their metabolic needs. Corals span a spectrum of trophic strategies and vary in their dependence on associated algal symbionts, with certain species capable of increasing heterotrophic feeding to compensate for the loss of autotrophic nutrition. As this ability can improve the likelihood of survival following marine heat waves and environmental disturbance, the continued threat of global and local stressors necessitates the investigation of trophic plasticity to determine coral responses to changing conditions. Here, we examined trophic strategy shifts between wet (high temperature and light) and dry (low temperature and light) seasons for seven genera of scleractinian corals by applying a Bayesian statistical model to determine the isotopic niches of paired coral hosts and their symbionts. Using a novel index (Host Evaluation: Reliance on Symbionts), trophic strategy was evaluated along a continuum of mixotrophy for each season. Three genera exhibited significant trophic shifts and were more heterotrophic in the dry season, likely as a mechanism to compensate for decreased symbiont functioning under lower temperatures and irradiance during these months. The magnitude of trophic plasticity varied across genera, and this pattern was positively correlated with global distribution. Together, our findings substantiate taxonomic differences in nutritional flexibility and provide support for trophic plasticity as a distinguishing trait for understanding coral biogeography.