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

The mechanisms of adaptation allowing resting freshwater mussels Anodontta cygnea to maintain their oxygen consumption (M_O2) constant when the O₂ partial pressure in the inspired water (Pl_O2) varied were studied at 13°C. Steady-state values of oxygen consumption and/or shell valve activity were determined at prefixed Pl_O2 for periods ranging from 2 to 15 days. Values of P_O2, O₂ concentration and acid-base status of arterial blood in the heart were determined after two days. M_O2 was maintained constant over Pl_O2 ranging from 35 to less than 1 kPa. At 0.3 kPa it decreased by 50%. Valves remained open (and M_O2 constant) most of the time even during a 4.5 day period at Pl_O2≈1.5 kPa. Between 35 and 1 kPa, blood P_O2 at the heart level can remain low and within a narrow range independent of Pl_O2. Blood P_CO2 increased at high Pl_O2 and decreased at low Pl_O2. 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 M_O2 when Pl_O2 varies. This common feature relies mainly on the ability to maintain P_O2 in the arterial blood at a value which is low and independent of Pl_O2. Hence, in terms of O₂, homeostasis of the milieu intérieur is accomplished.