They can’t stand the heat: marine heatwaves and bleaching impair stress responses and reproduction of a photosynthetic symbiont-bearing sea slug

Marine heatwaves (MHW) are intensifying, causing mass bleaching events in coral reefs. While their ecological and socioeconomic impacts are well documented, their cascading effects on specialized organisms remain understudied. Here, we investigate the impact that MHW and feeding on bleached prey have on a stenophagous tropical nudibranch, Berghia stephanieae , by analyzing its survival, reproduction, cellular stress response (CSR) and the photosynthetic activity of its endosymbionts. Thermal stress significantly reduced the number of eggs laid by B. stephanieae , while parental investment per egg was maintained. Moreover, exposure to a combination of MHW and bleached prey, impaired this nudibranch’s CSR and reduced survival, indicating a performance breakdown. These findings highlight the vulnerability of such specialized predators to MHW and the need to integrate them in marine conservation strategies. With extreme climate events becoming more frequent and prolonged, species featuring specialized diets can be amongst those paying the highest toll.