Fishes in Eelgrass Meadows Show Large Interspecific Differences in Thermal Acclimation to Marine Heatwaves

ABSTRACT Global warming is increasingly exposing shallow coastal habitats to extreme temperatures, with important consequences for associated fish communities. Eelgrass ( Zostera marina ), the most widespread seagrass in northern temperate regions, forms extensive meadows that provide nursery habitats, foraging opportunities, and refuge for a high diversity of coastal fishes. During intensifying marine heatwaves, these species are increasingly exposed to acute thermal stress. Upper thermal tolerance and short‐term acclimation capacity may determine which species persist or decline in warming eelgrass ecosystems, yet interspecific differences remain poorly understood. To address this gap, we experimentally exposed 12 wild‐caught fish species inhabiting eelgrass habitats in summer to two temperatures, Ambient (19°C) and Heatwave (23°C), representing current conditions on the Swedish Skagerrak coast (North Sea). We quantified critical thermal maxima (CT max ) as a proxy for acute upper thermal tolerance and assessed species' short‐term acclimation capacity following 5 days of exposure to a simulated marine heatwave. Most species increased their thermal tolerance, but both baseline thermal tolerance and acclimation response ratio (ARR) varied markedly among taxa. Juvenile Atlantic cod, whiting, and European plaice showed the lowest thermal tolerance and weakest ARR, suggesting limited capacity to rapidly buffer acute warming at the tested temperatures and a greater reliance on behavioral avoidance. In contrast, sedentary species such as gobies and pipefishes exhibited high thermal tolerance with moderate plasticity, whereas wrasses showed moderate tolerance but the strongest short‐term acclimation capacity. Temperature records from regional eelgrass meadows revealed summer conditions approaching or exceeding the upper thermal limits of several species examined. Together, these results demonstrate pronounced interspecific variation in thermal tolerance and short‐term acclimation capacity among fishes occupying eelgrass habitats. This suggests that species‐specific physiological limits and plasticity are likely to influence responses to marine heatwaves and reshape eelgrass fish assemblages under climate change.