Ectotherm’s vulnerability to climate change is predicted to increase with temperature variations. Still, translating laboratory observations of organisms’ heat stress responses to the natural fluctuating environment remains challenging. In this study, we used an integrative framework combining insights from the TDT curves and physiological reaction norms, to precisely capturePinctada margaritiferastage -specific thermal tolerance. On a second hand, this study aimed to explore applicability of the model of accumulation of thermal injury, by making in situ predictions at three contrasting sites across French Polynesia. By working with two life stages (early planktonic vs. spat), our study revealed an ontogenetic shift in lethal (CTmax) and sub-lethal (Tc) thermal limits, with higher vulnerability for early larval stages. Cumulative injury calculations resulted in thermal failure (100% injury) for larvae within 12 and 22 h, at the most ‘extreme’ site (Nuku Hiva; T°C > 38°C), and warm lagoon (Reao Atoll), respectively. While substantial damages accumulated in spats, when exposed to consecutive extreme tides (Nuku Hiva) for 8 days. Overall, our results revealed thatP. margarifierais living closer to their upper thermal limits than previously estimated, and inhabit environments where important reduction of settlement and heat stress are already occurring during warmest months.