COMPARATIVE SURVIVORSHIP OF SYMPATRIC NATIVE NORTH-AMERICAN GASTROPODS (ANGUISPIRA, MESODON, PHYSELLA, PLEUROCERA) AND AN INTRODUCED BIVALVE (DREISSENA) EXPOSED TO FREEZING TEMPERATURES

Seventy-four specimens from four genera of native gastropods were cool ed from 2.0 degrees C to -3.3 degrees C over a 3 hour period under lab oratory conditions. The taxa examined included two terrestrial pulmona tes (Anguispira alternata and Mesodon inflecta), a pulmonate known to occupy both permanent and ephemeral aquatic habitats (Physella integra ), and an aquatic prosobranch (Pleurocera canaliculatum). There was no mortality in the pulmonates, but 39% mortality occurred in the prosob ranch Pleurocera. Additionally, a sample of 43 zebra mussels (Dreissen a polymorpha), a non-native but recently introduced species, were aeri ally exposed to the same temperature cycle. Of the 43 mussels, 20 had been acclimated to 2 degrees C, and 23 acclimated to 15 degrees C. Mor tality occurred in both treatments (35% mortality in the 15 degrees C acclimated and 25% in the 2 degrees C acclimated) and did not differ s ignificantly between the two groups. No mortality occurred among contr ols.