In various phyla of marine invertebrates limited capacities of both ventilatory and circulatory performance were found to setthe borders of the thermal tolerance window with limitations in aerobic scope and onset of hypoxia as a first line of sensitivity toboth cold and warm temperature extremes. The hypothesis of oxygen limited thermal tolerance has recently been investigated infish using a combination of non-invasive nuclear magnetic resonance (NMR) methodology with invasive techniques. In contrastto observations in marine invertebrates arterial oxygen tensions in fish were independent of temperature, while venous oxygentensions displayed a thermal optimum. As the fish heart relies on venous oxygen supply, limited cardio-circulatory capacity isconcluded to set the first level of thermal intolerance in fish. Nonetheless, maximized ventilatory capacity is seen to supportcirculation in maintaining the width of thermal tolerance windows. The interdependent setting of low and high tolerance limitsis interpreted to result from trade-offs between optimized tissue functional capacity and baseline oxygen demand and energyturnover co-determined by the adjustment of mitochondrial densities and functional properties to a species-specific temperaturerange. At temperature extremes, systemic hypoxia will elicit metabolic depression, thereby widening the thermal windowtransiently sustained especially in those species preadapted to hypoxic environments.