The effect of suspended sediment on the suspension feeding and distribution of freshwater mussels (Bivalvia: Unionidae)
Unionid mussels are key organisms in freshwater systems in which they provide habitat for other organisms through their suspension feeding and burrowing activities. They represent one of the most imperilled freshwater taxa in part due to land use changes, which lead to increased total suspended solids (TSS). Such increases in TSS reduce mussel feeding rates. Despite this, unionids are often found in diverse communities in turbid rivers. Investigating how these animals exist in turbid rivers was the motivation of this thesis. Under no-flow conditions, clearance rates (CR; volume cleared of particles per unit time and individual) of juvenile (≥ 2 weeks old) and adult unionids declined with increasing TSS concentrations. The threshold TSS was ≥ 8 mg/L for adults and 2 - 10 mg/L for juveniles, depending on age and species. This response to TSS was conserved among sediment grain size fractions (mixed, clay, coarse silt) with the exception of a fine silt-sized fraction (5 – 38 µm diameter), which had little effect on the CR of adult mussels because of higher nutritional content (i.e., more fluorescent material, higher protein and lipid content). This contrasted with the prediction that clay-sized particles, which represent > 80% of suspended particles in rivers, were too small to affect CR. In a more ecologically relevant, flowing system, CR decreased with increased TSS but velocity modulated this reduction. The magnitude of the CR response was lower in mussels of the same species from a turbid river compared to those from a clear river, and the turbid river mussels had greater palp:gill area ratio, sturdier palp cilia, and denser laterofrontal cirri and cilia on the gills compared to clear river animals, which likely allowed them to process particles more efficiently. A juvenile rearing experiment revealed that the CR response of juveniles more closely matched their rearing condition than their natal population, providing evidence that the differences observed between the adult populations were due to a phenotypic plasticity. The results of this study will inform regulatory agencies in the management of water quality, and conservation efforts directed at the augmentation of unionid populations in the wild.