Bivalve shell growth from molecular to sclerochronological scale: Environment and intrinsic factors control increment deposition.

Biomineralisation of bivalve shells raises questions at the level of genes to the final calcified product. For the first time, gene expression has been studied in association with growth increment deposition in the mussel Mytilus galloprovincialis. A short-term experiment highlighted that biomineralisation genes exhibit a rhythm of expression consistent with the observed tidal increment formation. Long-term mark-recapture experiments were conducted in three Mediterranean environments and revealed that the mussel shells harbour complex incrementation regimes, consisting of daily, tidal and a mixed periodicity of ∼1.7 growth increment.d-1 formed. The latter is likely related to the local tidal regime, although the mussels were continuously submerged and exposed to a small tidal range. The pattern of growth increments shifted from mixed to daily in Mediterranean lagoon, and to tidal at sea, probably linked to biological clocks. Based on our results and the literature, a hypothetical model for mussel shell increment formation in various habitats is proposed.