Cumulative effects of successive marine heatwaves impair physiological resilience in the clam Ruditapes philippinarum.

Marine heatwaves (MHWs), discrete periods of anomalously warm seawater, are increasing in frequency, intensity and duration worldwide. However, most experimental studies investigate only one or two isolated short-term events. We simulated four successive heatwaves under current (+1.5°C; CHW) and future (+5°C; FHW) scenarios representative of projected conditions in the Bay of Brest, to assess their impact on the Manila clam, Ruditapes philippinarum. Successive heatwaves altered energy reserves with progressive carbohydrate depletion, selective remodelling of polar fatty acids, and altered hemocyte dynamics without inducing direct mortality, revealing a cumulative energetic stress across the four events. Burrowing performance was immediately impaired in FHW clams, while CHW clams exhibited a cumulative decline in burrowing capacity following repeated exposure. FHW clams exhibited elevated total hemocyte counts compared to CHW clams during heatwave exposure, followed by a significant decline during the final recuperation period. Following infection with Vibrio tapetis, a cold-water pathogen, a pronounced carry-over effect of thermal history emerged, with near-total mortality after four heatwaves, particularly under the FHW scenario. Our results suggest that even moderate heatwaves can generate cumulative physiological stress, while warmer events amplify dysregulation and disease susceptibility. These findings highlight that the ecological impact of marine heatwaves may be driven not only from upper temperature, but also from their increasing recurrence.