Acid-Base and Ionic Regulation, During and Following Emersion, in the Freshwater Bivalve, Anodonta grandis simpsoniana (Bivalvia: Unionidae)

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Abstract
Specimens of the boreal clam, Anodonta grandis simpsoniana were emersed at 10{deg}C for 6 days and then reimmersed for 24 h. The clams lost water at a rate of 1.6% total water per day. After 144 h of emersion, water weight had declined by almost 15%, while extracellular fluid (ECF) osmolality had increased 30% to 52 mOsm kg-1. Control levels were reattained after 6 h reimmersion. ECF Po2 declined rapidly in the first 24 h of emersion, but remained near 20 Torr for the full 6-day exposure. After an initial rapid fall, pH declined at a slower rate, reaching 7.494 +/- 0.037 (mean +/- SEM) at 144 h. Pco2 was elevated from 0.6 +/- 0.6 to 12.4 +/- 1.1 Torr after 96 h, but no further increase was noted. ECF [Ca] increased threefold to 13.1 +/- 0.8 mmol l-1, while [HCO3app] rose from 5.4 +/- 0.3 to a maximum of 12.9 +/- 0.8 mmol 1-1 after 144 h. ECF [Na] and [Cl] were not affected by emersion. On reimmersion, recovery was rapid, with pH, Po2 and Pco2 returning to control within 2 h, while [Ca] and [HCO3app] remained elevated until 24 h after reimmersion. A 1:1 stoichiometry between [Ca] and [HCO3app] existed throughout the emersion and reimmersion periods. In the absence of protein buffers, the fall in ECF pH was arrested by the mobilization of calcium carbonate, presumably from the shell. By 96 h emersion Pco2 and Po2 had stabilized, suggesting that diffusion gradients sufficient to allow limited gas exchange had been established.

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