RESPONSE OF AERIALLY-EXPOSED ZEBRA MUSSELS (DREISSENA-POLYMORPHA) TO SUBFREEZING TEMPERATURES

Zebra mussels (Dreissena polymorpha) were subjected to a temperature c ycle that exposed them to subfreezing temperatures over a three hour p eriod, similar to that which may be encountered in nature. Supercoolin g was clearly exhibited by 16 of 17 individuals with thermocouple prob es placed inside of the mantle. Cooling was continued for approximatel y 45 minutes after exotherms were produced to allow tissue freezing to occur. Two individuals with probes survived the cooling cycle, as did four of the 16 control animals without probes. Supercooling occurred between -1.8 C and -3.0 C ((x) over bar = -2.4, n = 16) and the reboun d temperature ranged from -0.1 C to -1.5 C ((x) over bar = -0.6, n = 1 6). One of the surviving zebra mussels implanted with a thermocouple p robe had a supercooling point of -2.4 C and a rebound temperature of - 0.6 C. This individual, cooled for an additional 45 minutes after exhi biting an exotherm, reached -1.7 C inside of the shell, indicating tha t at least some individuals can tolerate short-term freeze stress.