Roger A. Byrne

Acid-base and ionic regulation, during and following emersion, in the freshwater bivalve, Anodonta grandis simpsoniana (Bivalvia : Unionidae)

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.

The Effects of Aerial Exposure and Subsequent Reimmersion on Hemolymph Osmolality, Ion Composition, and Ion Flux in the Freshwater Bivalve Corbicula fluminea

Hemolymph osmolality increased twofold after 120 h aerial exposure at 95% relative humidity and 25° C and was correlated with water loss (r = 0.83). Blood sodium and chloride levels remained approximately constant throughout the period ofaerial exposure. Calcium increased threefold over 120 h emersion and was correlated with osmolality (r = 0.86). Unmeasured solute (presumably HCO₃) also increased. On reimmersion, osmolality returned to normal within 6 h. Hemolymph sodium and chloride levels were lower than controls after 3 d exposure and returned to control levels between 2-6h reimmersion. Hemolymph potassium and calcium concentrations dropped to control levels within 6h after return to pond water. Net ion fluxes in thefirst hour ofreimmersion were high and negative. Calcium and chloride initial netfluxes were highest at-41.6± 4.0 (mean ± SEM) and -23.9 ± 3.8 μEq (g dry tissue · h)⁻¹, respectively. Sodium andpotassium net fluxes were -65 ± 2.2 and -0.9 ± 0.2, respectively. Net ion fluxes decreased and by 5-16 h were not different from zero. There was no effect of aerial exposure on unidirectional sodium fluxes. Chloride eflux increased while influx was not affected. Aerial exposure stimulated the excretion of base equivalents on reimmersion. Some sodium and chloride may shift to intracellular space during emersion and return to hemolymph on reimmersion. Calcium and its anion were mobilized, presumably from the shell, for maintenance of body fluid acid-base balance.