Effects of temperature and pO(2) on the heart rate of juvenile and adult freshwater mussels (Bivalvia: Unionidae)

The heart rate of juvenile and adult Utterbackia imbecillis and Pyganodon cataracta was monitored during experimental manipulation of temperature 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 rate were assessed by subjecting mussels to a descending series of oxygen tensions (100, 75, 50, 25, 5 and 0% air saturation). Results indicated that the heart rates of juvenile and adult mussels are markedly affected by experimental temperature, with Q(10)'s approaching 4.5. Acclimation had no effect on adult mussels. However, juvenile U. imbecillis 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 decrease occurred at 0% air saturation. The differences in acclimation patterns exhibited by juvenile and adult mussels may reflect differences in the thermal conditions experienced by these two life-history stages; however, there are no data available that characterize the microhabitat of juvenile freshwater mussels. Utterbackia imbecillis appears to be more sensitive to low oxygen levels than P. cataracta, and the heart rate of juveniles of both species is more responsive to hypoxia than that of adults.