No abstract

Climate change challenges marine organisms by constraining their temperature-dependent scope for performance, fitness, and survival. According to the concept of Oxygen and Capacity Limited Thermal Tolerance (OCLTT), the overall thermal performance curve relates to an organism's aerobic power budget, its overall aerobic scope for growth, exercise, reproduction, and other performances. We hypothesize that physiological principles shaping tolerance in extant ecosystems have also been operative during climatic changes in the distant past. To compare response patterns in extant fauna and their palaeo-relatives, we started here by studying the metabolic background of performance in the European flat oyster Ostrea edulis at organismic and cellular levels, focusing on the acute thermal window and the metabolic changes towards upper lethal temperatures. We investigated the response of the oysters (pre-acclimated at 12 °C) to a short-term warming protocol (by 2 °C every 48 h) from 14 to 36 °C which we identified as the lethal temperature. At the organismic level, heart and filtration rates were recorded. Gill metabolites were studied by 1 H NMR spectroscopy to address thermal responses at the cellular level. Feeding activity by O. edulis (assessed by the filtration rates) was highest between 18 and 24 °C when overall energy expenditure (indicated by heart rate as a proxy for routine metabolic rate) was moderate. We conclude that this range reflects the thermal optimum of this species. Beyond 26 °C, the gill tissue of O. edulis became partly anaerobic, and cardiac dysfunction (arrhythmia) developed at 28 °C followed by an Arrhenius break point (30 °C). This mirrors performance constraints and indicates a wide temperature range of passive tolerance which may be a long-standing characteristic of ostreids supporting survival in extreme environments as well as during past and present climate oscillations.