Van Wynsberge S. auteur aut IRD Ladet E. auteur aut IRD Chevillard C. auteur aut IRD Le Gendre R. auteur aut IRD Maamaatuaiahutapu M. auteur aut IRD Moeroa M. auteur aut IRD Remoissenet G. auteur aut IRD Richmond T. auteur aut IRD Poltavtseef L. auteur aut IRD Tepapatahi T. auteur aut IRD Teaka R. auteur aut IRD Teara M. auteur aut IRD Tefau A. auteur aut IRD Petton S. auteur aut IRD Jullien S. auteur aut IRD Lafille M. A. auteur aut IRD Andréfouët Serge auteur aut IRD text journalArticle eng text/pdf born digital access Giant clams (Tridacna maxima) are abundant in French Polynesia semi-closed atolls, but the sustainability of the ecosystem services they provide has been challenged by recent mass bleaching events. Being able to accurately anticipate the risks of bleaching is needed to implement timely and effective mitigation strategies, but no method currently exists to assess bleaching risk from environmental data in these lagoons. To fill this gap, we used a hindcast approach to simulate the lagoon temperature experienced by giant clams at Reao Atoll (Tuamotu Archipelago) from 1993 to 2024. We then compared the reconstructed temperature time series with T. maxima bleaching states and size structures and densities during three El Ni & ntilde;o-induced heatwaves between 2016 and 2024. The 2016 event was the most stressful for giant clams, with 79.9% of individuals totally bleached. The 2017 and 2024 heatwaves induced lower bleaching than in 2016 despite an exposure to higher accumulated temperature stress levels, suggesting that giant clams' bleaching thermal threshold had increased after 2016. These observations were used to validate a new bleaching risk index (DHWT.maxima), inspired by NOAA's Coral Reef Watch coral bleaching indices, but optimised to account for the history of giant clam bleaching levels after several exposures to thermal stress. The newly designed index can be used to trigger local initiatives to mitigate the effect of bleaching on giant clam farming at Reao, and the method used in this study provides a framework to revisit the amount of thermal stress endured by symbiotic species in enclosed lagoon water bodies. specialized Marine heat waves Thermal threshold Tridacna maxima Degree heating week Tuamotu French Polynesia POLYNESIE FRANCAISE TUAMOTU 034 036 082 040 [Early access] [17 p.] 2026 0722-4028 https://www.documentation.ird.fr/hor/fdi:010096936 10.1007/s00338-026-02860-9 0722-4028 [F B010096936] https://www.documentation.ird.fr/hor/fdi:010096936 https://www.documentation.ird.fr/intranet/publi/2026-05/010096936.pdf Accès réservé (Intranet de l'IRD) IRD - Base Horizon / Pleins textes 2026-05-20 2026-05-20 fdi:010096936 fre