Acid-Base Balance during Emergence in the Freshwater Bivalve Corbicula fluminea

The Asian freshwater clam, Corbicula fluminea, was emersed at 23°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 790 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 suficient 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.