The phenology of the European flat oyster (Ostrea edulis) and its environmental drivers

The European flat oyster ( Ostrea edulis ) is an ecosystem engineer that structures reef habitats and is a major focus of restoration efforts across Europe, yet its biological rhythms in its habitat remain unknown. These rhythms synchronize physiology with environmental cycles, indicating ecological adaptation. Here, we investigated behavioral, physiological, and molecular rhythms in O. edulis , as well as their environmental drivers. Using high-resolution valvometry, we monitored valve activity and shell growth over 540-days in the German Bight (Helgoland), along with environmental variables such as photoperiod, temperature, underwater sound, and food availability. Monthly gene expression profiling of clock-related genes was also performed. O. edulis displayed coordinated daily, tidal, lunar, semilunar, and annual rhythms, mainly driven by photoperiod and temperature. This integrative chronobiological approach reveals a multiple rhythm system in O. edulis , highlighting both adaptive capacity and vulnerability to anthropogenic pressures, and providing key indicators for the conservation and restoration of oyster reef habitats.