According to the concept of oxygen limited thermal tolerance, performance on top of baseline energy expenditure is only possible within the temperature window limited by critical temperatures. The available excess of aerobic energy is shared between basic metabolism, muscular exercise, growth and reproductive processes like gamete production, which possibly take place at the same time in the body. Protein biosynthesis is the most important cellular process underlying organismal growth. We investigated protein synthesis in order to determine the thermal window of growth in lugworms from the North Sea and the Atlantic collected in different seasons and acclimated to different temperatures. Traditional growth experiments involve thermal acclimation processes due to long-term incubations. In contrast, our studies of protein synthesis rate in vivo elucidate the acute temperature dependent growth potential for each acclimation temperature. Comparisons of spring and summer animals revealed a shift in the growth performance window from 6-15°C (spring) to 10-18°C (summer), indicating a seasonal change in metabolism. At the same time, energy metabolism showed differing ratios of phosphotaurocyamine to inorganic phosphate in winter and summer acclimatised worms. Thermal tolerance windows and their seasonal shifts appear ecologically relevant in the light of global warming and climate change and might define the species distribution range. This study is part of a joined research project with Münster University (Animal Physiology) within the DFG priority program AQUASHIFT.