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Dryad

Riverine flow rate drives widespread convergence in the shell morphology of imperiled freshwater mussels

Keogh, Sean1; Pfeiffer, John2; Simons, Andrew3; Edie, Stewart2

Published Oct 17, 2023 on Dryad. https://doi.org/10.5061/dryad.dbrv15f75

Data files

Oct 17, 2023 version files 9.63 GB

Abstract

Frequent and strong morphological convergence suggests that determinism tends to supercede historical contingencies in evolutionary radiations. For many lineages living within the water-column of rivers and streams, hydrodynamic forces drive widespread morphological convergence. Living below the sediment-water interface may release organisms from these hydrodynamic pressures, permitting a broad array of morphologies and thus less convergence. However, here, we show that the semi-infaunal freshwater mussels have environmentally determined convergence in shell morphology. Using 3D morphometric data from 715 individuals among 164 Nearctic species, we find that species occurring in rivers with high flow rates have evolved traits that resist dislodgement from their burrowed position in the streambed: thicker shells for their body size, with the thickest sector of the shell being the most deeply buried. Species occurring in low-flow environments have evolved thinner and more uniformly thickened shells, corresponding to an alternative adaptation to dislodgement: increased burrowing efficiency. Within species, individuals also show increased shell thickness for their body size at higher flow rates, suggesting that ecophenotypy may, in part, be an important mechanism for establishing populations in new environments and thus evolutionary divergence in this highly imperiled invertebrate group.