Variability in cardiac activity of the bivalves Mytilus edulis and Modiolus modiolus

A noninvasive method was applied to study the heart rates of the White Sea bivalves Mytilus edulis and Modiolus modiolus in a long-term field experiment during the winter. No cardiac arrest and relatively high levels of cardiac activity (5.26 and 3.49 beats/min, respectively) were observed in both species. Oscillations of heart rate and signal amplitude with wavelengths of 3 to 6, 9 to 16, and 18 to 24 h were found. The oscillations of heart rate and cardiac signal amplitude occurred simultaneously and were connected with the tidal cycle only in the case of the short-term rhythms.