Evaluating Correlations Between Freshwater Mussel Shell Morphology and Valve‐Gaping Activity Across Riverine Conditions

Morphological features play a critical role in the ability of an organism to navigate its environment. Organisms living in rivers and streams must handle a spectrum of habitats that span from dynamic flows in headwater creeks to the regularly high flows of large rivers. Freshwater mussels (order Unionoida) are an interesting group of organisms that possess a variety of hypothesized shell morphology traits adapted to life across the stream size spectrum. Fusconaia flava (family Unionidae) is a species that exhibits a pattern of clinal variation, known as Ortmann's Law of Stream Position, where shell width increases with stream size. Although speculations have been made about the causes and consequences of Ortmann's Law of Stream Position, few experiments have been conducted to help explain why some freshwater mussel species exhibit this phenotype–environment association.

We conducted a comparative morphometric study and a fluvial experiment to identify how intraspecific mussel morphology and behaviour vary along a stream size gradient. We used micro‐computed tomography scanning to digitize shells from hydrologically diverse habitats, then measured the width, size and median thickness of each shell. Multiple regression was used to test the association between shell width and thickness while accounting for shell size, and linear regression models were used to describe the relationships between shell traits and hydrology. In a corresponding flume experiment, we measured valve‐gaping activity and observed presumptive burrowing behaviour of wild‐collected F. flava that exhibited ‘compressed’ (small‐river morph) and ‘inflated’ (large‐river morph) shell morphologies found in the upper and lower reaches of the Gasconade River in central Missouri, USA. We scaled flume flows to mimic observed low and high streamflows on the Gasconade River and used Hall sensors to record valve movements.

We found shell thickness and shell width were strongly associated with one another and increased with aspects of stream size (i.e., Strahler stream order, flow rate and catchment area). The flume experiment demonstrated the feasibility and mechanistic potential of using wild‐collected mussels in behaviour experiments. We successfully integrated flow scaling via Froude numbers and demonstrated the sensitivity of Hall sensors for continuous behavioural monitoring. Valve‐gaping activity of ‘compressed’ mussels did not appear to have an affinity to a specific flow regime, but visually they made significant spatial movements, whereas ‘inflated’ mussels exhibited pronounced changes in valve‐gaping activity under higher flow conditions but remained more or less stationary. However, our data were not capable of definitive behavioural comparisons and instead proved the value of this setup for future, larger‐scale experiments.

Given the ongoing and projected increase of droughts and catastrophic flood events due to climate change, our results provide insight into how morphological trait variation and valve‐gaping activity together allow individuals to respond to streamflow fluctuations. Additionally, our integration of comparative morphometrics and flume experiments provides a promising proof of concept for testing phenotype–environment associations.