Spawning is accompanied by increased thermal performance in blue mussels.

Climate change is causing extreme short-term warming with greater intensity and more frequent occurrence. Reproduction and subsequent recruitment of coastal ecosystem engineers, such as the blue mussel, may be impacted by the extreme temperatures because these vital functions are sensitive to the timing of short-term changes in abiotic factors. We exposed intertidal blue mussels, Mytilus edulis, to a thermal challenge from 10 to 29 °C using an ecologically relevant heating rate of 4 °C/h. We assessed their reproductive status by observing spawning activity and by analyzing histological sections of their gonad tissue. In addition, we monitored their heart rates and valve gaping behavior to determine their thermal performance. We identified three spawning groups: non-spawners that had mature gonads but did not release gametes, post-spawners that released mature gametes prior to the thermal challenge, and active spawners that released mature gametes during the thermal challenge. Across temperatures, active spawners had significantly higher heart rates and their heart rate peaked at the temperatures 3.9 and 3.2 °C higher compared to those of non-spawners or post-spawners, respectively. Post-spawners had significantly narrower valve gapes across temperatures compared to both other spawning groups. Hence, the metabolic response to warming strongly depends on the reproductive status, with active spawners experiencing increased thermal stress due to heightened metabolism, non-spawners showing heat-induced metabolic depression, and post-spawners adopting an energy-conserving strategy indicated by reduced gaping. Considered together, spawning during recurring short-term warming events may elevate mortality risk with potential consequences for the local biodiversity in a future climate.