Physiological and Behavioral Responses to Aerial Exposure in the Asian Clam, Corbicula Fluminea (Muller).

Aerially exposed, bivalve molluscs face opposing problems of preventing water loss, and maintaining metabolism. Because of the unpredictability of emersion duration, most freshwater bivalve families are capable of extensive emersion tolerance. Corbicula fluminea (Muller) is a relatively recent invader of freshwater and shares many physiological features with its estuarine ancestors. It might be expected to be intermediate in its emersion responses between those of more ancient freshwater forms, and those of estuarine clams. In air, C. fluminea displayed ventilatory behaviors which were associated with high rates of aerial O$\sb2$ uptake and CO$\sb2$ release. Normoxically exposed clams had bursts of heat dissipation unlike anoxically emersed clams. These bursts may have been associated with periodic aerial O$\sb2$ debt repayments. Tolerance to emersion was low (24, 72 and 248-342h at 35, 25 and 15$\sp\circ$C, respectively). High relative humidity stimulated mantle edge exposure. Clams could sense rates of drying and compensate behaviorally. Although hemolymph osmolality increased twofold after 120h emersion, only Ca and HCO$\sb3$ rose, while Na and Cl remained constant or declined. Na and Cl were redistributed within the clam. On reimmersion there were large losses of Ca and Cl, but hemolymph Cl remained close to control because of redistribution from cells. There was a progressive acidosis associated with duration of emersion, compensated by shell dissolution and buffering by CaCO$\sb3$. Compensation was incomplete and hemolymph pH declined from 7.90 to 7.14 in 72h. Bicarbonate, PCO$\sb2$ and total CO$\sb2$ also rose, but ventilatory release of CO$\sb2$ retarded the acidosis. There was an initial oxygen debt payment that increased with duration of emersion after 3 days exposure. Emersed clams did not accumulate ammonia and there was a lag time on reimmersion before maximal ammonia excretion rates were recorded, suggesting that clams can switch catabolic substrates to reduce toxic ammonia production. Although less tolerant of emersion than other freshwater clams, C. fluminea is very successful because of adaptations in its life history strategies. Many adaptations to emersion in C. fluminea are intermediate between those of estuarine and other freshwater clams, but some seem to be novel, recently evolved responses