Freshwater bivalve shells represent a valuable but underutilized archive for reconstructing past environmental conditions and biological responses. In northern New York State, these organisms are widespread across the freshwater ecosystems of the Adirondack Mountains and the adjacent St. Lawrence River Lowlands. This also region presents a change in bedrock geology that significantly influences surface water chemistry. The southern Adirondack Mountains are underlain by poorly buffered, highly metamorphosed Precambrian crystalline bedrock, contributing to more acidic aquatic environments. In contrast, the St. Lawrence Lowlands are characterized by Lower Paleozoic sedimentary formations, including limestone, sandstone, and shale, resulting in higher pH conditions. The continuous distribution of freshwater bivalve populations across this geochemical gradient provides a unique opportunity to compare representatives of the taxon Elliptio complanata based on their shell shape, growth rates, and shell porosity across the pH regimes.

Material was collected from 2 rivers in the Adirondack Mountains and the St. Lawerence lowlands in northern New York. Shell thin sections provide the data for Bertalanffy growth curves across the study area. Morphometric landmark analysis of shells shapes across all localities was performed and their micro density calculated. Our results showed that individuals from more acidic environments produce smaller, narrower, and more porous shells. These morphological differences are associated with a life history strategy characterized by accelerated early growth followed by reduced growth rates during later developmental stages. This pattern suggests that bivalves in more acidic conditions may prioritize rapid early shell formation to mitigate the possibly corrosive effects of low pH environments. These findings may be indicative of suboptimal conditions for biomineralization.

This study demonstrates that shell characteristics in freshwater bivalves can serve as proxies for environmental stressors, such as acidification, and could function as an early warning system for detecting the vulnerability of freshwater ecosystems by identifying characteristic shell shapes of stressed populations. Unlike studies limited to living organisms, shell records provide a longer temporal archive extending beyond the lifespan of one individual. The findings raise important questions about the limits of phenotypic plasticity in these bivalve populations and suggest that conservation strategies, such as the translocation of acid-tolerant lineages from placed like the Adirondack mountains, may be a tool to enhance population resilience in increasingly acidified habitats.