Variation in Freshwater Mussel Shell Sculpture and Shape Along a River Gradient

It has been hypothesized that variation in shell shape and sculpture in freshwater mussels from streams is related to hydrologic variability. Using quantitative sampling, we found smooth-shelled species were more common in upstream portions of the St. Croix River, Minnesota and Wisconsin, which has coarser substrate while species with sculptured shells were more common downstream and in finer substrate. For four species distributed throughout the length of the river, individuals tended to have shells that increase in obesity (width/length) and relative height (height/length) downstream. These results support suggestions that mussels with sculptured or obese shells may have an anchoring advantage in areas of high discharge while smooth-shelled or slim forms may reburrow more readily after dislodgement in hydraulically flashy environments. One species that showed increased obesity downstream had juveniles with similar shell shape at all locations suggesting phenotypic plasticity in shell shape. The juvenile and adult valve shape for the other three species examined varied among locations making it is less clear whether the differences noted are genetically fixed or phenotypically plastic. Understanding the complex interplay between shell shape and sculpture and disturbance regimes in rivers may provide useful information when developing conservation plans for these vulnerable animals. While both shell ornamentation and shape vary along river length, and may be correlated with substrate anchoring and stream flow, other factors such as behavior and physiology also contribute to mussels' success in maintaining their location in a river.