Time in shells: Complex interaction between biological clock and biomineralisation in Mytilus galloprovincialis

The growth of bivalve shells is neither homogeneous nor continuous in time, resulting in the formation of growth patterns that correspond to the alternation of growth lines and increments deposited at regular intervals of time. The control of periodic increment formation is poorly understood and several hypotheses have been proposed. It has been proposed that environmental factors directly impact shell growth patterns, although it occasionally fails to adequately explain the observed shell growth patterns. The present study investigates the alternative hypothesis that the process of shell biomineralisation is controlled by biological clocks. This study demonstrates the existence of a functional circadian clock in M. galloprovincialis, as evidenced by molecular and behavioural results. Core circadian clock genes and biomineralisation genes have been observed to be expressed in the same cells of the mantle as revealed by in situ hybridisation experiments. However, the expression of core circadian clock genes and biomineralisation genes tested in situ and in aquaria exhibited different rhythmic profiles. This finding suggests that the clock does not directly activate the expression of the targeted biomineralisation genes in the mantle. Nevertheless, a significant rhythm of expression of biomineralisation-related genes was observed in mussels reared under free-running conditions, revealing the endogenous nature of the rhythm. The present study suggests that biological clocks play a role in controlling shell biomineralisation in M. galloprovincialis, although the precise underlying mechanism remains to be elucidated.