Nautilus can survive for long periods in water of verv low oxygen content. Below a PO2 of around 25 mm Hg activity becomes intermittent, with brief periods of jet propulsion punctuating longer (typically 15-20 min) periods at rest. During these inactive periods ventilatory movements cease and the heartbeat slows to one or two cycles of verv low amplitude per minute. The onset of an active period is heralded by an increase in heartbeat frequency and amplitude and a resumption of visible ventilatory movements. The large blood volume and verv high oxygen affinity of the blood pigment apparently ensure that the animal accumulates sufficient oxygen during the resting intervals to fuel the brief active periods. Recovery, even after prolonged exposure to near anoxic conditions, is very rapid if the animal is exposed to well-aerated water. These capacities are not only related to a strategy of defence by withdrawal but would also allow Nautilus to exploit environments where the low oxygen content might limit competition from most fish and crustaceans. It is argued that a Nautilus-like physiology and behaviour could well have contributed to the success of extinct ectocochleate forms, living in oceans that were, by and large, less well oxygenated than now. The downward extension of oxygen-rich water and progressive elimination of hypoxic regions from the continental shelves during the Mesozoic may have contributed to the extinction of the ectocochleates by opening up the hitherto hypoxic environments, which they were adapted to exploit, to the more strictly aerobic high metabolic rate competitors and predators that eventually replaced them.
