Investigations of the physiological mechanisms shaping latitudinal adaptation and seasonal acclimatization to temperature are carried out to go further into the question of how marine invertebrates and their ecosystems respond to climate change. While thermal responses can be found at various levels of organisation we expect sensitivity to be highest at the organismal level, the animals balance of oxygen demand and supply. This balance is composed of energy consuming processes like metabolic maintenance, growth, reproduction, muscular exercise and others on the demand side, and of oxygen providing processes like ventilation, blood oxygenation, circulation, tissue oxygenation and cellular respiration on the supply side. Rising temperature leads to a mismatch situation causing an increased oxygen demand. The organism has to react either by increasing the rate of supply processes or by reducing energy consuming processes that are not essential for survival. The capacity of the supply system is limited and when pejus temperatures are reached, it is not able to meet demand any more, blood oxygenation decreases. Long-term warming beyond pejus temperatures thereby leads to reduced performance (e.g. growth, reproduction, muscle exercise) with likely severe ecological consequences (decreased abundance, local extinction, shift in distribution). Performance optima and the width of thermal tolerance windows and their location on the temperature scale were quantified investigating standard metabolism, growth and muscular exercise on the demand side and ventilation, haemoglobin properties and tissue oxygenation on the supply side in populations of the lugworm Arenicola marina. The data demonstrate latitudinal differences between these windows and their seasonal shifts and thus a close relationship between the local climate regime and the degree of thermal specialization of the respective population.