July-to-August seawater temperature recorded at 18 m depth at the “3PP” station, La Ciotat, near Marseille, South of France (43.163°N; 5.599°E) using in situ autonomous sensors during the 2022 MHW, in the two previous years (grey dotted lines), and during the reference 2003 MHW (orange dotted line). Only one record per day is shown; IMG 1467: Two living sponge species, Ircinia oros and the bath sponge Spongia officinalis, the latter with necrotic spots on its surface (September 7, 2022) - Author Thierry Pérez IMG 1688: The same two sponges after five days (September 12, 2022), S. officinalis is dying, covered by a white bacterial veil. Author Thierry Pérez P9216598: A once rich and healthy population of S. officinalis at 10 m depth, September 21, 2022; Arrows point to S. officinalis dead skeletons left attached to the rocky wall; Author Pierre Chevaldonné

Note published in Global Change Biology