THE PHYSIOLOGICAL ECOLOGY OF THE ZEBRA MUSSEL, DREISSENA-POLYMORPHA, IN NORTH-AMERICA AND EUROPE

The zebra mussel, Dreissena polymorpha (Pallas), was introduced into N orth America in 1986. Initial North American (N.A.) studies suggested that physiological responses varied between N.A. and European populati ons. However, literature review indicates agreement on most aspects of physiological adaptation including: respiratory responses; hypoxia/an oxia tolerance; salinity limits; emersion tolerance; freezing resistan ce; environmental pH limits; calcium limits; starvation responses; and bioenergetic partitioning. The main differences among N.A. and Europe an mussels appear to be elevated upper thermal limits and temperatures for optimum growth among N.A. populations. N.A. zebra mussels probabl y originated from the northern shore of the Black Sea in the warmest p ortion of the mussel's European range. However, most European physiolo gical data come from northern Europe where populations may be adapted to colder temperatures. Alternatively, N.A. research suggests that mus sels may have a capacity for seasonal temperature acclimatization such that responses recorded in wanner N.A. waters may be different from t hose recorded in northern Europe even after short-term laboratory accl imation. Studies of genetic variation and physiological response among European and N.A. D. polymorpha populations are required to elucidate the basis for physiological differentiation. Recently evolved D. poly morpha has poor resistance adaptations compared to unionacean and spha eriid bivalves with longer freshwater fossil histories. Poor resistanc e adaptations make it less suited for stable habitats, instead, its hi gh fecundities, early maturity, and rapid growth are adaptations to un stable habitats where extensive resistance adaptations are of little v alue.