Long-term increases in shell thickness in Elliptio complanata (Bivalvia: Unionidae) in the freshwater tidal Hudson River

Abstract

The shells of freshwater molluscs are highly plastic in response to environmental and ecological conditions, which can affect ecological interactions and ecosystem functioning. We tested for changes in relative shell thickness in the unionid bivalve Elliptio complanata in the freshwater tidal Hudson River over 1991–2017, a period of changing water chemistry and predation pressure. Shells of both juveniles (shells <20 mm long) and larger animals became substantially (c. 10%–25%) and significantly heavier (for a given shell length) over the study period. This parallels previously published results for the non-native zebra mussel Dreissena polymorpha in the Hudson. Increased shell thickness during the study period could be explained by rising pH, alkalinity, and temperature, which increased calcite saturation and favoured the precipitation of calcium carbonate shells, as well as a response to greater predation by shell-crushing predators. Because the factors that affect shell thickness (e.g. calcium carbonate saturation and presence and abundance of predators) are changing rapidly in many lakes and rivers, similar large changes in shell thickness may be occurring in fresh waters around the world, affecting ecological interactions and ecosystem functioning.