FRESHWATER RIVER MUSSEL GAPING RESPONSE BEHAVIOR TO INCREASED WATER VELOCITY AND TOTAL SUSPENDED SOLID CONCENTRATIONS

Freshwater mussels are key river ecosystem engineers, but it is unclear how native mussel community dynamics affect near-bed sediment dynamics or how mussels respond to changing sediment loads. Previous work shows a greater decline in mussel diversity and numbers in the Minnesota River Basin, which has higher total suspended solid (TSS) concentrations than the St. Croix River Basin. To investigate the role of elevated TSS concentrations on mussel behavior we ran four 48-hour flume experiments at St. Anthony Falls Laboratory at the University of Minnesota. We monitored mussel movement and used gaping, the open/close behavior, as a proxy for feeding of sixteen mussels in control (Mississippi River water) and treatment (increased sediment) sections under two velocity regimes. Monitoring was conducted using gape sensors attached to mussel shells of two species with different shell structure.

We identified three common patterns present in mussel behavior. “Long events (hours)” occur when mussels were completely closed, “movement events” occur when a mussel is physically moving on top of or within the bed sediment, and “short events (seconds)” occur when mussels are either filter feeding or excreting pseudofeces, particles filtered but expelled back into the water column before digestion. While there were fewer differences between the total time in long events and time between movement closure events between varying conditions, mussels generally exhibited an increased frequency of short closure events when exposed to more intense stressors. The time between short event occurrences for the average Amblema plicata mussel exposed to a mean TSS of 93 mg/L and 0.6 m/s water velocity was almost 40 minutes shorter than those exposed to a mean TSS of 13 mg/L and 0.25 m/s water velocity. Similarly, the average time between short events for Lasmigona complanata mussels exposed to greater stressors was 10.5 minutes less. This trend seen in time between short events is likely influenced by an increased pseudofeces production, requiring quick abrupt valve closings to expel the material. Our experiments provide evidence that geomorphic environmental stressors have quantifiable impacts on mussel gaping behavior.