7-3 Examining Changes in Shell Morphology and Microstructure From the Mid-20th Century to Present in an Ecologically Critical Mussel Species From Southeastern Alaska, USA

Session: Earth Life Sciences across the Cordillera (Posters)

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Increased carbon dioxide emissions from human activities are driving major changes in ocean chemistry, including warming waters, intensifying ocean acidification, and disruptions to upwelling and nutrient regimes. These stressors have profound impacts on marine organisms, particularly calcifying species that rely on shell formation for survival. Marine bivalve mollusks, such as the California mussel (Mytilus californianus Conrad, 1837), record ambient environmental and climatic conditions through biomineralization, making them valuable archives of environmental change across time. This study examines decadal changes in shell morphology, growth banding, mineralogical layering, crystallographic orientation, and trace element composition in the M. californianus, a dominant foundation species of wave-exposed rocky intertidal zones along the eastern North Pacific. We analyzed ~70 specimens collected from southeastern Alaska between 1947 and 2024, integrating archival museum collections with modern field-collected material to evaluate changes in calcification patterns over nearly eight decades. Shell morphology exhibited strong allometric scaling, with width increasing with length across all periods, while more recent shells clustered toward smaller, narrower forms relative to mid-20th-century specimens. Calcification declined significantly through time (ANOVA: F_4,64 = 29.67, p = 5.77 × 10^-14), with 2020s shells showing markedly lower calcification indices than the 1940s–1950s, 1960s, 1970s, and 1990s (Δ = −0.29 to −0.44), whereas standardized inner calcite thickness did not differ between archival and modern samples. Growth banding and shell thickening showed contrasting size relationships, with dark–light band pairs only weakly related to length but inner calcite thickness increasing moderately with shell size. Ongoing microstructural analyses are expected to document temporal changes in calcite crystal orientation within the species’ unique secondary inner prismatic calcite layer, including increased crystal disorder and greater inter-individual variability within the shell. Exploratory trace element analysis of magnesium, strontium, and barium profiles may serve as proxies for changing ocean temperature and carbonate chemistry. Relative to subtropical populations in Southern California and Baja California, Mexico, these results could suggest that high-latitude populations of M. californianus are experiencing amplified climate-driven stressors, leading to reduced shell growth and compromised shell integrity. Such changes have the potential to propagate through rocky intertidal ecosystems and affect the ecosystem services historically supported by this species, highlighting the importance of assessing latitudinal vulnerability under continued climate change.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

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