The Circulatory Response to Acute Hypoxia in Octopus

Octopus vulgaris can regulate its oxygen uptake in a closed respirometer down to a Poo2 of less than 70 mmHg. As the tankwater Poo2 falls the hearts slow down. Pulse amplitudes and mean pressures fall in the afferent branchial vessels and in the dorsal aorta. Despite behavioural changes - expansion of the interbrachial web and extension of the arms - that might imply this, the proportion of the total oxygen uptake attributable to cutaneous respiration (less than 13%) does not alter as the external Poo2 falls. The response of the hearts to a low Poo2 is not affected by severing the nerve supply from the central nervous system, or by removal of the heart ganglia. It is evidently determined by oxygen lack and not by the accumulation of CO2 or other metabolites, since the same effects are achieved by placing the animals in water where the Poo2 has been reduced by boiling. The conclusion that regulation does not depend upon circulatory responses to hypoxia is considered in the light of recent work on the changes in blood oxygen affinity associated with acute hypoxia in cephalopods.