THE cephalopod mollusc Nautilus is often called a living fossil because its multichambered shell resembles that of extinct forms from the early Palaeozoic era. Living at depths of 100-300 m, the animals not only encounter zones of low oxygen concentration, but also exploit them for refuge or feeding(1,2). Despite some modest recruitment of anaerobic sources of energy, Nautilus is able to survive severe bouts of hypoxia, mainly through its prodigious capacity for aerobic metabolic rate suppression. Here we show that the hypometabolic, hypoxic animal conserves energy further by means of prolonged ventilatory and circulatory pauses, during which time the blood, having a comparatively high oxygen affinity, is apparently loaded at the venous side, across the superficially located and voluminous vena cava. Under these highly arrested conditions, a significant fraction of the animal’s aerobic metabolic rate can be accounted for by a slow ’metering out’ of the O-2 store contained in the shell, indicating that the buoyancy chambers of Nautilus may occasionally subser ve the role of a ’SCUBA tank’, The internal shell morphology and siphuncular arrangement seen in the fossilized remains of ammonites suggest that similar processes may have occurred.