EFFECTS OF TEMPERATURE AND PO(2) ON THE HEART-RATE OF JUVENILE AND ADULT FRESH-WATER MUSSELS (BIVALVIA, UNIONIDAE)

The heart rate of juvenile and adult Utterbackia imbecillis and Pygano don cataracta was monitored during experimental manipulation of temper ature and pO(2) Animals that had been acclimated to 15 and 25 degrees C for 1 week were exposed to an ascending series of temperatures (10, 15, 20, 25 and 30 degrees C). The effects of oxygen tension on heart r ate were assessed by subjecting mussels to a descending series of oxyg en tensions (100, 75, 50, 25, 5 and 0% air saturation). Results indica ted that the heart rates of juvenile and adult mussels are markedly af fected by experimental temperature, with Q(10)'s approaching 4.5. Accl imation had no effect on adult mussels. However, juvenile U. imbecilli s exhibited inverse acclimation; whereas, juvenile P. cataracta fit a typical pattern of acclimation (Presser's Type IVA). When exposed to a descending series of oxygen tensions, adult U. imbecillis maintained a constant heart rate until a significant increase occurred at 25% air saturation, followed by a significant decrease at 5 and 0%. Juvenile U. imbecillis maintained a constant rate until bradycardia occurred at pO(2)'s Of 5 and 0% air saturation. Juvenile and adult P. cataracta, exhibited similar patterns, with sustained rates until a significant d ecrease occurred at 0% air saturation. The differences in acclimation patterns exhibited by juvenile and adult mussels may reflect differenc es in the thermal conditions experienced by these two life-history sta ges; however, there are no data available that characterize the microh abitat of juvenile freshwater mussels. Utterbackia imbecillis appears to be more sensitive to low oxygen levels than P. cataracta, and the h eart rate of juveniles of both species is more responsive to hypoxia t han that of adults.