A functional morphological classification for North American freshwater mussels: associations between morphology and environmental parameters across spatial scales

Trait-based approaches have emerged as a general framework that translates species-specific knowledge to understand the processes driving patterns of diversity and distributions. Morphological traits are relatively easy to measure and can provide information on organism-environment interactions and community structure due to their close linkage to ecological function and habitat partitioning. Freshwater mussels (Family: Unionidae) are a diverse (~360 North American species) and endangered group of organisms. Mussels display great interspecific morphological variation potentially yielding broad ecological implications. We aimed to modify quantitively an existing shell morphological classification system by combining size, shape, and sculpturing data using a robust data set of 1362 individuals representing 64 species spanning a broad cross section of the diverse North American freshwater mussel fauna. Using multivariate techniques, we classified species into morphological classes based on trait similarities hypothesized to explain species distributions and habitat associations. We then tested how well the classification system predicted trait-environment relationships using quantitative mussel survey data with paired environmental data collected at three spatial scales [river (km), reach (40-150 m), patch (0.25 m²)]. Mussel species clustered into six different morphological classes based on sculpturing, shape, and body size traits. We found associations between morphological classes and environmental parameters at each spatial scale. The modified classification explained more variation in community distribution as predicted by abiotic variables than previous frameworks. Our study underscores the value of morphological traits in predicting species distributions and understanding mechanisms of community assembly and we provide a foundation for fellow researchers to expand our morphological classification. This knowledge has significant implications for mussel conservation and management, as it helps identify suitable habitats that can guide reintroduction strategies through incorporating multiple spatial scales, a broad representation of species and geographical distribution and a wide suite of morphological traits.