PATTERNS OF ORIENTATION IN UNIONIDS AS A FUNCTION OF RIVERS WITH DIFFERING HYDROLOGICAL VARIABILITY

As unionids can become dislodged with high flows, it may provebeneficial for an individual to minimize its exposure to theflow. This can be accomplished by either burrowing as deep aspossible or orienting in a way that effectively reduces thedrag exerted on the mussel by the flow. The patterns of orientationwere examined in unionids with respect to hydrological variability.The orientation of mussels to flow was measured at four sitesalong an event river (hydro-logically variable) and a stableriver (hydrologically stable). Burrowing depth was measuredat a reference site in each river. Most individuals in bothriver types were oriented with their siphons pointing upstream.Mussel orientation differed significantly between the two rivertypes with mussels in the event river orienting more parallelto the flow than those in the stable river. Mussels in the eventriver were significantly larger than those in the stable riverbut the size of a mussel did not determine its orientation withina river. Burrowing depth did not differ for mussels betweenevent and stable rivers. The observed differences in orientationsamong river types are likely a function of differences in thepattern of orientation of the mussel community as a whole, withineach drainage. This burrowing behaviour may be an attributethat enhances the adaptations mussels have for remaining burrowedin the sediment. (Received 2 December 1996; accepted 24 February 1997)