Acid-Base-Balance During Emergence In The Fresh-Water Bivalve Corbicula-Fluminea

The Asian freshwater clam, Corbicula fluminea, was emersed at 23-degrees-C under three conditions: humid air, N2 hypoxia, or air with valves clamped or bound shut. Under all conditions, clams became acidotic during emersion. In air, hemolymph pH declined from 7.90 to 7.14 in 72 h. Hypoxic clams displayed a similar decline in pH, but clams with valves clamped became acidotic more rapidly. Hemolymph CO2 content (CCO2) and PCO2 rose with duration of aerial exposure in all treatment groups. However, hemolymph of bound clams had significantly higher CCO2 and PCO2 than that of either air- or N2-exposed bivalves after 48 h emersion. Although hemolymph PO2 declined with emersion time in all groups, there was no effect of exposure condition, suggesting that ventilatory behaviors did not affect hemolymph PO2 or that the effects were masked by rapid utilization of inspired O2. Hemolymph calcium and osmolality increased with emersion time and were greatest in clams with clamped valves. Presumably, shell calcium carbonate was mobilized to provide bicarbonate to buffer the acidosis, while valve gaping, mantle edge exposure, and other ventilatory behaviors served to retard the progress of the acidosis by ventilatory loss of CO2. However, these behaviors were not sufficient to maintain acid-base balance in C. fluminea. The inability of C. fluminea to maintain or restore acid-base balance during short emersion periods may account for this species' low tolerance of aerial exposure and is further evidence of its recent invasion of freshwater habitats.