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Jellyfish blooms might be driven by the alterations in seawater temperature (SWT) associated with climate change. The physiological responses of jellyfish to changing SWT, however, are poorly understood. Therefore, we asked the question: how do sudden changes (+/- 6 degrees C) in SWT affect the physiological performance of the jellyfish Cassiopea sp.? We measured the changes in mitochondrial cellular respiration (i.e., in term of the electron transport system (ETS) activity), superoxide dismutase (SOD) activity, and lipid peroxidation (LPO) to assess the jellyfish's physiological performance. In acute treatments (2 h), ETS increased only in response to cooling (to 20 degrees C) while SOD remained unchanged. In response to chronic treatment (2 weeks), ETS, SOD and LPO increased, while body mass decreased in response to cold (20 degrees C). In contrast, the heat-treated (32 degrees C) jellyfish did not increase their metabolic demands nor show signs of oxidative stress (OS). Moreover, they gained body mass. Because chlorophyll-a remained unchanged in all chronic-treated jellyfish, the cold-induced OS is more likely due to cellular respiration, not photosynthesis. Overall, Cassiopea sp. seems more sensitive to decreases in SWT then to increases. Therefore, Cassiopea sp. might benefit from the future projected rises in SWT, which could result in increased population abundance and an expansion in geographic distribution. Overall, these finding add new physiological evidences on jellyfish tolerance and might be used as a framework for further studies aiming at better understanding of jellyfish physiology.