Global warming trends in the marine environment currently lead to poleward shifts in the distribution of marine fauna along European coastlines indicating limited thermal tolerance of affected species and potential loss of their southernmost populations. The present study analyses the degree and limits of thermal specialisation in various populations of a key species of the intertidal zone, the lugworm Arenicola marina, which is exposed to highly fluctuating conditions in temperature, salinity, pH and oxygen levels during the seasonal as well as the tidal cycle. Thermal windows of tolerance and exercise performance were compared in three populations from the Russian White Sea, the German North Sea and the French Atlantic coast. Digging experiments in natural marine sediments quantified performance capacity and its optimum temperature. Temperature dependent ventilation and respiration were analysed in lugworms dwelling in artificial self-perfused burrows. Haemoglobin oxygen affinity was examined for an evaluation of population specific adaptations in the oxygen supply system. The results reflect a shift of both the thermal window and the thermal optimum towards higher temperatures with decreasing latitude. This shift was accompanied by a reduction of performance amplitude, i.e. the absolute number of digging periods, and a widening of the distance between critical temperatures, i.e. the total aerobic window of thermal tolerance. © 2009 Elsevier B.V. All rights reserved.