Mollusc shell periostracum as an alternative to tissue in isotopic studies

Recent studies have used carbon and nitrogen stable isotope ratios of preserved soft tissues to examine historical changes in trophic patterns of aquatic ecosystems. A limitation in this application is the difficulty in finding specimens for primary consumers, which can act as a surrogate for basal food sources in determining the trophic status of higher consumers. The availability of preserved soft tissues of invertebrate primary consumers is often limited in museum and archival collections; hard parts such as mollusc shells, however, are often abundant because of their ease of storage. We used stable isotope ratios of carbon and nitrogen to determine if there was a relationship between the isotopic composition of the periostracum of the shell and soft body tissue of freshwater molluscs. We found a significant correlation between (1) periostracum and tissue of freshly collected freshwater mussels, (2) ethanol‐preserved adductor muscle tissue and dry‐preserved mussels from museum collections, and (3) ethanol‐preserved tissue and periostracum of gastropods from museum collections. The predictability of these relationships enhances the capacity to track changes in trophic complexity over time and responses of food webs to natural and anthropogenic environmental perturbations.