The lugworm Arenicola marina is a key organism from the intertidal zone, which is very abundant and shapes the ecosystem by bioturbation. The lugworm is distributed over a wide latitudinal range and exposed to widely fluctuating temperature conditions during seasonal as well as diurnal (tidal) cycles. Animals are specialized to adjust to ambient climate conditions as well as climate variability as evidenced from patterns of latitudinal adaptation and seasonal acclimatisation of thermal tolerance windows. While thermal responses can be found at various levels of organisation we expect climate sensitivity to be highest at the organismal level, the animals balance of oxygen demand and supply. Quantification of the resulting organismal performance range is relevant in the light of global warming and climate change, as exposure to temperatures beyond the performance window likely leads to fitness losses with the consequence of decreased local abundances and eventually a shift in geographical distribution. Performance optima, the width of thermal tolerance windows and their location on the temperature scale were quantified by investigating basic metabolism, growth and muscular exercise on the demand side and ventilation, haemoglobin concentration and oxygen affinity on the supply side in populations of Arenicola marina from the French Atlantic coast, the German North Sea and the Russian White Sea. Performance optima shift to higher temperatures at decreasing latitudes. Seasonal shifts to higher temperatures occur during summer acclimatisation emphasizing a close relationship between the local climate regime and the degree of thermal specialisation of the respective population.