Investigating controls of shell growth features in a foundation bivalve species: Seasonal trends and decadal changes in the California mussel

Marine bivalve mollusk shells can offer valuable insights into past oceanographic variability and seasonality. Given its ecological and archeological significance, Mytilus californianus (California mussel) presents the opportunity to examine seasonal and decadal changes recorded in its shell over centuries to millennia. While dark-light growth bands in M. californianus shells could be advantageous for reconstructing past environments, uncertainties remain regarding shell structure, environmental controls of dark-light-band formation, and the amount of time represented by a dark-light pair. By analyzing a suite of M. californianus shells collected in 2002, 2003, 2019, and 2020 from Bodega Bay, California, we describe the mineralogical composition; establish relationships among the growth band pattern, micro-environment, and collection season; and compare shell structure and growth band expression between the archival (2002-2003) and modern (2019-2020) shells. We identified three mineralogical layers in M. californianus: an outer prismatic calcite layer, a middle aragonite layer, and a secondary inner prismatic calcite layer, which makes M. californianus the only Mytilus species to precipitate a secondary calcite layer. Within the inner calcite layer, light bands are strongly correlated with winter collection months and could be used to reconstruct periods with moderate, stable temperatures and minimal upwelling. Additionally, modern shells have significantly thinner inner calcite layers and more poorly expressed growth bands than the archival shells, although we also show that growth band contrast is strongly influenced by the micro-environment. Mytilus californianus from northern California is calcifying differently, and apparently more slowly, than it was 20 years ago. Copyright