Environmental, genetic, and age-induced control explains the continent-scale shell variability in a short-lived freshwater mussel

Freshwater mussels are characterized by a high degree of shell variability but the factors controlling spatial conchological shifts are poorly understood. Here, we use the Duck Mussel Anodonta anatina (Linnaeus, 1758) (Unionidae) as a target species to address the influence of environmental, ontogenetic, and genetic variables on the shell shape variation in a short-lived, generalist freshwater mussel at the continental scale. Samples from 70 localities, situated in Europe, North and Central Asia, and the Middle East, were examined. The structural equation model (SEM) indicates that the shift from ovate to round morphotype in Anodonta anatina throughout Eurasia is associated with climatic and ontogenetic variables (SEM: R2 adjusted = 0.567, Q2 = 0.292, SRMR = 0.08; climate and latitude: β = 0.514, P < 0.000; median age: β = 0.357, P < 0.001). The Mantel tests reveal that the interpopulation morphological distances correspond to genetic differentiation but with small correlation coefficients. The relative shell convexity in Anodonta anatina populations is controlled ontogenetically (median age). Our results highlight that correlation between the shell shape of the Duck Mussel and climate is likely a product of phenotypic plasticity with potentially unique morphologies associated with genetic differentiation in the AZOV lineage.