The marine environment confronts its inhabitants with a wide variety of O2 concentrations, as well as with fluctuations of small scale local O2 availability over time. To cope with the unpredictable availability of oxygen in their next environment, marine invertebrates have evolved behavioural and physiological strategies, including mitochondria with a capacity for anaerobic energy production, sulphide oxidases and alternative endoxidases (AOX) branching of the mainstream electron transport pathway.These AOX are insensitive to sulphide inhibition and have lower Km values for oxygen than cytochrome oxidase, to allow for oxyconforming respiration during high tissue oxygenation. Taken together, all these traits evidence the early evolution of hypoxia tolerant marine animals in an atmosphere low in oxygen and mildly sulphidic, which did not allow to exclusively rely on oxidative phosphorylation. The other side of the coin is a high sensitivity of many species to elevated and even normoxic oxygen saturation, which has led to investigate their susceptibility to oxygen radical forming processes in cells and mitochondria. In this talk we will disclose behavioural and cellular/mitochondrial strategies employed by marine invertebrates to adjust tissue Po2 low and constant and alleviate oxidative stress under environmental stress situations and to highlight important differences with mammalian mitochondria.