Eelgrass-associated fishes show large interspecific differences in thermal acclimation to marine heatwaves

Global warming is increasingly exposing shallow coastal habitats to thermal extremes, with important consequences for the fish species they support. Eelgrass (Zostera marina), the most widespread seagrass in the Northern Hemisphere, provides nursery habitats and foraging opportunities for a high diversity of temperate fishes. However, light limitation is compressing eelgrass depth distribution to shallower waters, increasing exposure of these habitats and their associated fish assemblages to thermal stress during intensifying marine heatwaves. Persistence in warming eelgrass habitats may therefore depend on species-specific differences in thermal tolerance and the capacity for rapid thermal acclimation among fishes. However, interspecific variation in acclimation capacity within shared habitats remains poorly understood. To address this gap, we experimentally exposed twelve wild-caught eelgrass-associated fish species to ambient (19 ºC) and heated (23 ºC) temperatures, representative of current summer conditions of the Swedish west coast (Skagerrak, North Sea). We quantified critical thermal maxima (CTmax) as a proxy for upper thermal tolerance and assessed species’ short-term acclimation capacity following five days of exposure to a simulated marine heatwave in the laboratory. Most species increased CTmax, but both baseline thermal tolerance and acclimation capacity varied markedly among taxa. Juvenile Atlantic cod, whiting and plaice showed the lowest thermal tolerance and weakest acclimation responses, suggesting limited capacity to physiologically buffer acute warming and a greater reliance on behavioural avoidance. In contrast, sedentary species such as gobies and pipefishes exhibited high thermal tolerance with moderate plasticity, while 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 acclimation capacity among eelgrass-associated fishes. This indicates that ongoing warming and marine heatwaves are likely to reshape eelgrass fish assemblages with implications for coastal food-web structure and functioning.