The circadian rhythms of valve movements in the mussel <Emphasis Type="Italic">Mytilus galloprovincialis</Emphasis>

The long-term (10–30 day) continuous recording of valve movements in the mussel Mytilus galloprovincialis was carried out in the laboratory under nearly natural conditions. Fourier analysis revealed the circadian (close to the diurnal) rhythm of the valve activity, the phase of which is readily shifted by a shift in the beginning of the daytime, since the light regime is one of the main factors determining the circadian rhythm. The circadian rhythm was manifested in the daily dynamics of mussel valve activity: in the daytime, mussels hold their valves closed more often than at night. This behavior may be a protective response, namely the “shadow reflex”: mussels close their valves upon a sudden decrease in illumination, thus protecting themselves from possible predators. Circadian activity can mask a mussel’s response to environmental pollution; therefore, regular valve closure should be taken into account in early warning systems such as “MusselMonitor®” with a correction for the season of the year, time of day, and other factors.