Abstract
Abstract. 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.
Highlights
Marine bivalve shells offer a complex yet valuable record to explore questions about paleo- and modern seasonal extremes, since many bivalve species can record ambient conditions as they calcify (Jones and Quitmyer, 1996; Wanamaker Jr. et al, 2006; Welsh et al, 2011; Schöne and Gillikin, 2013; Trofimova et al, 2021)
We identified three mineralogical layers in M. californianus: an outer calcite layer with faint indistinguishable banding; a thin nacreous aragonite middle layer; and an inner calcite layer that grows inward, precipitating dark–light-band pairs
Within the genus Mytilus, the inner calcite layer is unique to M. californianus and may be a useful layer for the reconstruction of extreme conditions and determination of the season of collection due to strongly expressed growth banding, the contrast between dark–light bands is variable and dependent on tidal position and habitat type
Summary
Marine bivalve shells offer a complex yet valuable record to explore questions about paleo- and modern seasonal extremes, since many bivalve species can record ambient conditions as they calcify (Jones and Quitmyer, 1996; Wanamaker Jr. et al, 2006; Welsh et al, 2011; Schöne and Gillikin, 2013; Trofimova et al, 2021). Resolved shell growth features allow for the approximation of historic baselines of temperature variability, the comparison of inferred paleotemperature extremes to modern temperature ranges, and the prediction of climatic changes on organisms and ecosystems, which are typically controlled more strongly by extremes (Sydeman et al, 2014; Poloczanska et al, 2016; Mellin et al, 2016) than they are by average conditions. During calcification, both environmental and biological factors influence shell characteristics and chemistry differently depending on the species (Table 1).
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