EXAMINING DECADAL SCALE CHANGES IN THE SHELL OF A FOUNDATION BIVALVE MOLLUSC SPECIES

Biological responses to rising ocean temperatures are of particular concern in marine calcifying taxa. An ecologically and culturally important marine calcifier is Mytilus californianus, a mussel species that plays a critical role in structuring and maintaining intertidal ecosystems along the West Coast of North America. Previous studies of M. californianus from Washington and southern California presented evidence for rapid and recent shell thinning, increased crystallographic disorder, and fluctuations in the mineralogical composition, but M. californianus growing in northern California is understudied despite its potential for disciplines ranging from paleoceanography to ecology. Here, we characterize calcification patterns of M. californianus shells collected in 2002, 2003, 2019, and 2020 from Bodega Bay, California to explore recent changes in the growth band pattern, mineralogical layering, and shell thickness. Analysis of 40 shells showed that the Bodega Bay mussels contained 14 percent more dark bands than light growth bands overall, indicating that mussels in northern California spend more of their lives experiencing hindered rather than normal growth. We also show that contrast between dark and light growth bands has decreased by 52 percent, suggesting a drastic weakening in the visual expression of growth banding. The thickness of the inner calcite layer has also decreased significantly relative to shell length since 2002-2003. Our results suggest that M. californianus from northern California are calcifying differently, and apparently more slowly, than they were 20 years ago, indicating that change has occurred, and is likely ongoing, over a decadal scale in a foundation species.