Thermal tolerance windows were investigated in lugworms Arenicola marina (L.) from the North Sea, the English Channel and the Atlantic during winter and summer seasons. We adopted the concept of oxygen- and capacity-limited thermal tolerance and tested a variety of methods for their Suitability to identify characteristics of the respective thermal window, such as critical, pejus (lat.: getting worse) and optimum temperatures (T-c, T-p and T-opt), as well as temperature-dependent performance. First, the effect of acute temperature changes on oxygen consumption and pumping activity was examined in artificial burrows ventilated by lugworms collected in winter and summer from North Sea mudflats (acclimation temperatures 5 and 10 degrees C). Second, the window of acutely temperature-dependent growth was evaluated by following the integration of uniformly labelled 1 C-L-phenylalanine into proteins of the cuticulo-muscular tube, using nuclear magnetic resonance spectroscopy, possibly providing access to T, values. Third, seasonal changes in haemoglobin half-saturation oxygen tension (P-50) and haemoglobin oxygen status in vivo were studied in lugworms from the North Sea population. T, values were defined by use of oxygen consumption measurements, whereas pumping activity, growth performance and tissue oxygenation allowed determination of T,,,,. Our results show that the thermal window of actively ventilating lugworms is narrower in winter than in summer. The acute optimum temperature of growth performance was tilled towards the lower T, and likely shifts with acclimation. Methods were found suitable to identify the acute thermal window of oxygen-limited thermal tolerance in the lugworm. Consecutive comparison of populations during various seasons in a latitudinal cline should thus lead to a better understanding of the impacts of climate change on marine ecosystems.