Behavioral and Physiological Responses to Emersion in Freshwater Bivalves

SYNOPSIS. Littoral lentic and shallow lotic freshwater habitats are unpredictable in periodicity and duration of shore emersion. As a result, freshwater bivalves have evolved extensive capacities to withstand prolonged emersion. Valve movement behaviors allow emersed bivalves to control rate of water loss while maintaining at least partial aerial gas exchange; these behaviors are affected by environmental variables such as temperature and relative humidity. Aerial oxygen uptake is associated directly with valve ventilatory behaviors and mantle edge exposure. Such behaviors are often phasic, indicative of oxygen “debt” payment. Lacking effective hemolymph buffer, respiratory acidosis during emersion is compensated by shell carbonate stores allowing hemolymph PCO2 to rise to levels facilitating diffusion of CO2 to the environment. During emersion, hemolymph calcium can increase four fold while Na and Cl are tightly regulated. Ammonia production ceases in emersed bivalves. It resumes on reimmersion, indicative of heavy reliance on non-protein catabolism during emersion. Some emersion adaptations of freshwater species appear to be modifications of those displayed by intertidal and estuarine bivalves, while others appear independently evolved to allow survival of the extreme emersion periods associated with life in shallow freshwaters.