Juvenile unionid mussels disperse in the water column after detachment from their host fish. The settling velocity (ws) of juvenile mussels is an important component of their dispersion in the water column but has not been measured for unionid mussels. The goal of our study was to measure the ws of juvenile mussels in the laboratory and to examine how ws varied within and among related species. The ws of Actinonaias ligamentina and Ptychobranchus fasciolaris were significantly lower (2.4 ± 0.1 mm/s vs 2.5 ± 0.1 mm/s, respectively) than those of the larger-sized Lampsilis fasciola and Epioblasma triquetra (4.2 ± 0.2 mm/s vs 4.6 ± 0.2 mm/s, respectively). Overall, ws increased with juvenile size, but considerable variation (∼10×) was found within species. Observations indicated that foot movement of juvenile mussels was responsible for reductions in ws, and this behavior may provide a potential mechanism for habitat selection at small-spatial scales. Observed ws differed considerably from ws predicted from Stokes' law using empirically determined shell size and density (ρ = 1.22 ± 0.003 g/cm3 for A. ligamentina), which indicates some of the limitations in predicting ws from size measurements.
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