Experimental investigation of turbulent flow over live mussels

Unionids have been described as ecosystem engineers capable of affecting the local food web as well as the hydrodynamics, passively and actively. In this study, we perform particle image velocimetry measurements to characterize the flow around a live Amblema plicata using natural sediments as tracer particles under two specific flow rates and orientations i.e., facing upstream and downstream, to understand the interaction between the organism and flow surrounding it. The behavior of the mussel is also quantified under all the above experimental conditions using a hall-effect gape sensor that captures valve motion. The flow measurements show clear variations between the two orientations tested, with the presence of a low velocity region behind the mussel extending beyond the tested field of view for the upstream orientation. In contrast, the flow in the wake of the downstream orientation recovers very quickly to the incoming flow strength. The inspection of the vorticity contours around the mussel also highlights clear differences between orientations, with the structure of a shear layer under the upstream orientation changing to a region of concentrated vorticity near the siphons for the downstream cases. The variation of the net flux through the mussel along with the frequency of partial closures captured by the gape sensor measurements illustrate a potential added bioenergetic cost to the mussel feeding when facing downstream, especially at higher flow speeds.