Freshwater pearl mussels Margaritifera margaritifera L. were exposed to air for 7 days, then immersed in circumneutral water (pH7.85) or acidified water (pH 5.25) for 5 days. Mantle fluid pH and composition were monitored throughout. The mussels were observed to gape periodically when in air. Periodic gaping permitted aerial gas exchange such that mantle fluid Pcoco2 and dissolved oxygen concentration stabilized at levels about twice and half, respectively, those of immersed mussels. During emersion, a dilute carbonate buffer equilibrium was established in the mantle fluid, involving reactions with CaCO3 reserves and, simultaneously, aerial release of CO2. Aerial CO2 release was sufficient to shift the carbonate buffer equilibrium in the alkaline direction, resulting in a significant alkalosis of mantle fluids during air exposure. Mantle fluid characteristics returned to initial (time zero) values within 3 days of immersion in circumneutral water (pH7.85). When immersed in acid water (pH5.25), the mussels were able to maintain a sizeable gradient between mantle fluid pH and ambient water pH. Regulation of mantle fluid pH in acid water did not involve any isolation reaction (valve closure), but rather environmental protons were buffered at the expense of CaCO3 reserves. Net calcium transfer, the difference between calcium uptake and loss, was shifted in the negative direction by decreases in water pH.

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