Freshwater mussels as a tool for reconstructing climate history

Abstract

Sclerochronology provides an important and widely used tool to examine annual environmental patterns in marine systems, but few similar tools have been developed to establish ecological indicators in freshwaters on a seasonal scale. Previous work using marine mussels as ecological indicators have practiced shell ashing, acetate peels, and thin sectioning, all of which destroy the specimens. We studied the external annual rings of three freshwater mussel species with clear annual bands from the River Medway, UK, in order to non-invasively investigate the relationship between banding patterns, growth parameters, location, and seasonal water temperatures. We tested the accuracy and precision of this method through repetition of measurements and the reproduction of results through separate agers. Overall, mussels living downstream had higher length-at-age curves, asymptotic lengths (L∞), and growth constants (k) than those living upstream. In Unio pictorum and Unio tumidus, there was a negative relationship between asymptotic lengths and growth constants, indicating that larger asymptotic lengths took more time to reach. Unio tumidus had the highest asymptotic lengths and the lowest growth constants across sites while Unio pictorum had the lowest asymptotic lengths and the highest growth constants across sites. Anodonta anatina showed simultaneously increasing asymptotic lengths with increasing growth constants and had the highest overall lengths of all three species. Summer water temperatures had the largest positive effect on annual growth, followed by spring and winter water temperatures, while autumnal temperatures had a negative impact on growth. The findings of this study suggest that through their annual banding, freshwater mussels can be used as tools to document historical water temperatures, especially in the warmer months of spring and summer, and can therefore serve as powerful indicators of spatial and temporal patterns in water temperature. Such tools can help infer palaeoclimatic conditions from fossil and subfossil shells, establish baselines for understanding future climatic change, and support conservation efforts aimed at protecting temperature-sensitive taxa.