The shell morphology of freshwater mussels (Unionidae) varies in response to resource abundance and habitat conditions. Complex shell variations have historically led to over-description (splitting) of many nominal species, hampering the identification and mapping of historical biogeography of highly diverse, but endangered North American unionids. Ortmann’s (1920) Law of Stream Position posits adaptive ecophenotypic plasticity of freshwater mussel shell in response to stream position: individuals in larger rivers produce heavily sculptured and/or more inflated shells than conspecifics in headwaters to prevent dislodgement from the substrate by currents. We develop a novel morphometric measure of the volumetric ratio of nacre : soft tissue of unionids along river gradients using structured light 3D scanning, to quantify resource allocation in response to local flow regimes. Using specimens in historical collections from the Wesleyan University, CT and the Academy of Natural Sciences, PA, we found that species producing different types of shell modification allocate resources differently. Fusconaia subrotunda
(Ohio River watershed) demonstrates a positive correlation between river discharge rate and resource allocation to shell-building: individual mussels in the Ohio River allocate significantly more nacre to build an inflated shell than F. subrotunda
in smaller tributaries. Cyclonaias pustulosa
from the large Wabash River produces highly pustulated shells with slightly but significantly higher nacre volume relative to soft tissue volume, than those from the Rocky River tributary.
Our results suggest that pustules are more economical to produce than inflated shells, even though both shell modifications augment chances of survival in large rivers, potentially explaining the prevalence of sculptured species in large rivers. Our revisit of Ortmann’s Law after 101 years using modern morphometric techniques has the potential to be used as a proxy to extrapolate palaeoenvironments in which fossil bivalves lived in, to examine pre-historic flow regimes and to discover previously unknown changes to shell morphology.