BEHAVIORAL AND PHYSIOLOGICAL-RESPONSES TO EMERSION IN FRESH-WATER BIVALVES

Littoral lentic and shallow lotic freshwater habitats are unpredictabl e in periodicity and duration of shore emersion. As a result, freshwat er bivalves have evolved extensive capacities to withstand prolonged e mersion. Valve movement behaviors allow emersed bivalves to control ra te of water loss while maintaining at least partial aerial gas exchang e; these behaviors are affected by environmental variables such as tem perature and relative humidity. Aerial oxygen uptake is associated dir ectly with valve ventilatory behaviors and mantle edge exposure. Such behaviors are often phasic, indicative of oxygen ''debt'' payment. Lac king effective hemolymph buffer, respiratory acidosis during emersion is compensated by shell carbonate stores allowing hemolymph P-CO2 to r ise to levels facilitating diffusion of CO2 to the environment. During emersion, hemolymph calcium can increase four fold while Na and Cl ar e tightly regulated. Ammonia production ceases in emersed bivalves. It resumes on reimmersion, indicative of heavy reliance on non-protein c atabolism during emersion. Some emersion adaptations of freshwater spe cies appear to be modifications of those displayed by intertidal and e stuarine bivalves, while others appear independently evolved to allow survival of the extreme emersion periods associated with life in shall ow freshwaters.