Limited capacity for rapid acclimation to marine heatwaves in coastal invertebrates.

Marine heatwaves are increasing in frequency and intensity worldwide and represent a growing threat to coastal ecosystems and the animals inhabiting them. The vulnerability of marine species to these events depends on several factors, including upper thermal tolerance, the thermal sensitivity of physiological performance, and the capacity for rapid physiological acclimation. We examined upper thermal tolerance and short-term acclimation capacity in 10 mobile macroinvertebrate species inhabiting shallow coastal ecosystems in the Northeast Atlantic. Individuals were exposed either to ambient thermal conditions at 15°C or to a simulated short-term marine heatwave of 5 days at 5°C above ambient temperature. We then measured critical thermal maxima (CTmax) to quantify thermal limits and acclimation responses. Across species, CTmax increased significantly following heatwave exposure, showing that these coastal invertebrates can undergo rapid physiological adjustment. However, the magnitude of the acclimation response was modest, indicating that short-term plasticity provides only limited protection against acute thermal extremes. These limited acclimation responses suggest that some coastal invertebrates may remain vulnerable to intensifying marine heatwaves, particularly taxa with narrow thermal safety margins. Our findings provide an ecologically relevant assessment of thermal performance in coastal invertebrates and contribute to predictions of how nearshore communities may respond to accelerating ocean warming.