Effects of Juvenile Settling and Drift Rates on Freshwater Mussel Dispersal

Abstract

Freshwater mussels play important roles in ecosystems and are globally imperiled. Adult mussels are sedentary and dispersal is as larvae attached to fish hosts and as juveniles in stream drift. Understanding juvenile dispersal is important for understanding the patchy distribution of adult mussels and for conservation. We propose the location in which a juvenile mussel ends up on the streambed is determined by a combination of the juvenile's settling velocity, current speed, and streambed topography. We conducted a laboratory experiment to quantify the settling velocities of juveniles of four mussel species and of polymer microparticles, later utilized as surrogates for juveniles. We performed a flume experiment where we measured drift distances of microparticles at three low flow velocities over gravel and over a simulated mussel bed (gravel combined with mussel shells). Lastly, we measured drift distances of microparticles at five sites in two small rivers under summer low flow conditions. There were no significant differences in settling velocities among the four mussel species, live juveniles sank at a significantly lower rate than dead juveniles, and there were no significant differences in settling velocities between live juveniles and microparticles. The distances microparticles drifted in the flume increased with flow velocity and were shorter over the simulated mussel bed than over gravel. In the field most microparticles were recaptured near their release locations. In this study, juvenile mussels had limited dispersal potential with most drifting < 10 m. This limited dispersal should result in many juveniles settling within mussel beds, areas where habitat conditions are favorable for survival and growth, and may contribute to the patchy spatial distribution of mussel beds in rivers.