Long‐term increases in shell thickness in <i>Elliptio complanata</i> (Bivalvia: Unionidae) in the freshwater tidal Hudson River

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