Hydrodynamic and suspended sediment effects on valve behavior and glycogen content in two freshwater mussels

Freshwater mussels are key stream ecosystem engineers but are threatened by changing environmental conditions and human disturbances. Increasing flows and suspended sediment concentrations from land-use change and increased storm intensities could be contributing to observed declines in mussel density and diversity, especially in the intensively agricultural midwestern United States. We investigated the role of these factors on mussel behavior and stress in 2 freshwater mussel species (Amblema plicata [Say, 1817] and Pterosyna complanata [Barnes, 1823], formerly Lasmigona complanata) by measuring valve gape behaviors, movement, and glycogen content in a flow-through flume supplied by Mississippi River water. The experimental design included 4 combinations of water velocities (0.25 and 0.60 m/s) and elevated total suspended solid (TSS) treatments (background river water: ~13 mg/L; elevated TSS concentrations: ~60 and ~100 mg/L) in 48-h experiments. We identified 3 patterns in mussel gape behavior that varied between species: 1) valve closure periods—completely closed valves, 2) feeding periods—largely open valves with partial closures associated with feeding or excreting, and 3) adjustment periods—rapid valve activity associated with horizontal or vertical movement. At the end of each experiment, we collected mantle tissue clips and analyzed glycogen content as a proxy for individual stress response. Water velocity and species identity affected most stress endpoints (valve closure, frequency of partial valve closures during feeding, total movement, and glycogen content), but TSS only affected the frequency of partial valve closures during feeding. Across species, mussels displayed longer valve closures, higher frequency of partial valve closures during feeding, and lower glycogen content in higher water velocities. More frequent partial closures in higher water velocity and TSS conditions indicate that mussels were filtering more often and likely excreting pseudofeces. These results highlight how incorporating hydrodynamic conditions can improve understanding of mussel stress, helping resource managers make informed decisions regarding this imperiled faunal group.