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

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.

Conspicuous shell shape plasticity across lake-stream habitats in a freshwater mussel (Pyganodon grandis)

“…Experimental results from this study and others (Hinch et al 1986; Tuttle-Raycraft and Ackerman 2020) show that freshwater mussels can conditionally express their anatomical features in response to environmental cues and phenotypic plasticity is likely a ubiquitous feature of mussels given strong ecophenotypic variation in other species (Grier 1920; Ortmann 1920; Ball 1922; Agrell 1948; Graf 1998; Balla and Walker 1991; Zieritz and Aldridge 2009; Keogh et al In Press). Given its high and predictable morphological variation, Pyganodon grandis may be an ideal model system to study phenotypic plasticity and moreover, the functional genomics of shell shape in bivalves.…”

“…Hydrology is the most obvious environmental difference between lake and stream field sites and quantification of within-species ecophenotypic relationships between morphology and water velocity (Balla and Walker 1991), flow rate (i.e. discharge) (Graf 1998; Keogh et al In Press), Reynolds number (i.e. hydraulic energy) (Simeone et al 2022), and Strahler stream order (Ortmann 1920; Keogh et al In Press) suggest that freshwater mussels reliably respond to hydrologic changes.…”

“…discharge) (Graf 1998; Keogh et al In Press), Reynolds number (i.e. hydraulic energy) (Simeone et al 2022), and Strahler stream order (Ortmann 1920; Keogh et al In Press) suggest that freshwater mussels reliably respond to hydrologic changes. However, the zoo population, reared in basket enclosures within a lake, had drastically different morphology with more ‘stream-like’ relative heights and widths (Supplementary Figure 1).…”