INTERPRETING GROWTH BAND PATTERNS IN THE CALIFORNIA MUSSEL FOR USE AS AN ENVIRONMENTAL AND BIOLOGICAL ARCHIVE

Marine bivalve shells can serve as archives of seasonal cycles, oceanographic variability, and human impacts over the Holocene. Given its history as an archaeologically significant species, the California mussel (Mytilus californianus) presents a unique opportunity to examine and compare environmental and biological changes recorded in its shell at various sites along the California coast through time. M. californianus shell structure comprises an outer calcite layer, a thin middle layer of aragonite, and an inner calcite layer with dark-light growth banding. While dark-light bands could serve as markers of time or indicators of environmental conditions, the temporal resolution and controls of dark vs. light band formation remain uncertain. Here, we investigate the relationships between growth band pattern (thickness, color, expression, frequency), season of collection, and environmental conditions (temperature, rainfall, upwelling regime) among mussels from two sites with high-resolution, long-term instrumental records: Bodega Marine Reserve in northern CA and San Diego in southern CA. Using a suite of modern and archival shells from both sites collected over different seasons from 1996-2020, we compare average growth band thickness, average number of dark-light couplets, season of collection, and most recent band color. Relationships established here will help to 1) determine the timing and resolution of a growth band (and constrain species- and site-specific ‘growing windows’), 2) identify distinct growth trends between northern vs. southern California mussels over the past two decades, and 3) utilize dark (slow growth) bands as a physiological proxy for periods of anaerobiosis, or non-ideal growing conditions. Given that growth bands represent both environmental and physiological events and processes, the shell of the California mussel could be advantageous for both paleoceanographic and paleoecological reconstruction once the dynamics of shell formation are understood in detail.