Changes of the Cardiac Activity of the Intertidal White Sea Mussel Mytilus edulis L. during Exposure and Immersion

Cardiac activity of mussels Mytilus edulis L. was studied in situ at the lower and middle intertidal zone of the White Sea. Heart rate was recorded in September, when water and air temperatures were above zero, and in November, when air temperatures were below zero. As expected, cardiac activity (both the heart rate and the amplitude) of the mussels decreased sharply at low tide, sometimes down to a complete cardiac arrest. In September, decrease in cardiac activity started well in advance of complete exposure. Recovery of cardiac activity also started in advance of immersion at high tide. In both cases, so-called overshoots were observed; that is, both the heart rate and the amplitude exceeded average values (by 2–3 beats min−1 and 0.3–0.4 units). The heart rate of mussels in September was much higher than in November (10.0 vs. 6.0 and 11.0 vs. 5.8 beats min−1 for the lower and middle intertidals, respectively). No rhythmic changes of cardiac activity were noted when the mussels were placed under constant laboratory conditions, which indicates an exogenous nature of the rhythm observed in nature. The results are discussed in terms of adaptation of mussels to the drastic fluctuations of environmental factors in the intertidal zone.