Patterns of Orientation in Unionids as a Function of Rivers With Differing Hydrological Variability

As unionids can become dislodged with high flows, it may prove beneficial for an individual to minimize its exposure to the flow. This can be accomplished by either burrowing as deep as possible or orienting in a way that effectively reduces the drag exerted on the mussel by the flow. The patterns of orientation were examined in unionids with respect to hydrological variability. The orientation of mussels to flow was measured at four sites along an event river (hydro-logically variable) and a stable river (hydrologically stable). Burrowing depth was measured at a reference site in each river. Most individuals in both river types were oriented with their siphons pointing upstream. Mussel orientation differed significantly between the two river types with mussels in the event river orienting more parallel to the flow than those in the stable river. Mussels in the event river were significantly larger than those in the stable river but the size of a mussel did not determine its orientation within a river. Burrowing depth did not differ for mussels between event and stable rivers. The observed differences in orientations among river types are likely a function of differences in the pattern of orientation of the mussel community as a whole, within each drainage. This burrowing behaviour may be an attribute that enhances the adaptations mussels have for remaining burrowed in the sediment.