Acroporids in northwestern Philippines with varied thermotolerance host similar photosymbionts

Coral reefs worldwide are threatened by rapid warming of the oceans, yet many corals persist despite thermal stress. Reefs in northwestern Philippines, which are frequently exposed to elevated temperatures (29–30°C, mean monthly maximum), present an opportunity to examine inter-colony variation in thermotolerance and its correlation with Symbiodiniaceae, the coral’s microalgal symbiotic partner. In this study, we assessed the thermotolerance of individual colonies of three Acropora species, A. digitifera , A. millepora , and A. cf. tenuis , from a reef in Anda, Pangasinan, Philippines. Thermotolerance varied within and among species. ITS2 metabarcoding revealed that the corals host four closely related strains of Cladocopium patulum (formerly referred to as “type C3u”). However, inter- and intra-specific differences in thermotolerance did not show strong correlation with symbiont composition. These findings suggest that while Symbiodiniaceae communities may contribute to heat resilience in corals, they do not solely explain the marked difference in thermotolerance among individuals.