Oxygen uptake and the mode of energy production in Sipunculus nudus L. were determined at different oxygen levels by means of direct and indirect calorimetry. Oxygen consumption declined linearly with decreasing ambient POO2 A similar decrease in heat production was observed down to a POO2 of 8.66 kPa. At lower oxygen tensions, a discrepancy between aerobic and total heat production indicated the onset of anaerobic metabolism. The occurrence of the critical POO2 between 8.66 and 2.66 kPa was confirmed by estimation of anaerobic end products in the body wall musculature. The contributions of aerobic and anaerobic metabolism to total ATP production were determined at a POO2 of 2.66 kPa and were found to be 48 and 52%, respectively. Measured heat dissipation under extreme hypoxia (POO2 nominally zero) (21.5±3.5 mJ h−1 g−1) agreed with the enthalpy changes calculated from the rates of formation of anaerobic end products (17.9±4.7 mJ h−1 g−1). For the sake of redox balance maintenance, saturation of fatty acids was assumed; this would be accompanied by an additional heat production of 3.3 mJ h−1 g−1, so that the total calculated enthalpy change amounted to 21.2 mJ h−1 g−1

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