How is O2 consumption maintained independent of ambient oxygen in mussel Anodonta cygnea?

The mechanisms of adaptation allowing resting freshwater mussels Anodonta cygnea to maintain their oxygen consumption (MO2) constant when the O2 partial pressure in the inspired water (PIO2) varied were studied at 13 degrees C. Steady-state values of oxygen consumption and/or shell valve activity were determined at prefixed PIO2 for periods ranging from 2 to 15 days. Values of PO2, O2 concentration and acid-base status of arterial blood in the heart were determined after two days. MO2 was maintained constant over PIO2 ranging from 35 to less than 1 kPa. At 0.3 kPa it decreased by 50%. Valves remained open (and MO2 constant) most of the time even during a 4.5 day period at PIO2 approximately 1.5 kPa. Between 35 and 1 kPa, blood PO2 at the heart level can remain low and within a narrow range independent of PIO2. Blood PCO2 increased at high PIO2 and decreased at low PIO2. Data are compared to previous results in crayfish and in wels (sheat-fish). On the basis of data similarity it is proposed that these three animals exhibit the same basic strategy for maintaining resting MO2 when PIO2 varies. This common feature relies mainly on the ability to maintain PO2 in the arterial blood at a value which is low and independent of PIO2. Hence, in terms of O2, homeostasis of the milieu intérieur is accomplished.