No abstract

Unionid mussels from clear‐water rivers are often found in turbid waters in which their feeding and reproductive efficiency should be impaired. We examined the feeding response of Lampsilis siliquoidea from a clear and turbid river in response to increased concentrations of total suspended solids (TSS) and water velocity in a funnel chamber and a recirculating flow chamber. Four TSS concentrations (0, 5, 20, and 100 mg L−1) and four velocities (0, 0.05, 0.15, and 0.25 m s−1) were used to create 16 experimental conditions corresponding to nine TSS flux (concentration × velocity) levels. TSS flux significantly affected clearance rates (CR); however, the relationship was complex. Increased TSS led to lower CR in mussels from clear and turbid river at all velocities; however, increases in velocity reduced the magnitude of the decline with respect to TSS. Overall, the turbid river mussels were less affected by increased TSS than those from the clear‐water rivers. The mussels from the turbid river also differed anatomically (i.e., greater palp:gill ratio; number of cilia per cirri; number of cirri cm−1; and thickness of palp cilia, and less cilia per unit area of palp) from the clear river animals. It is likely that these anatomical differences allow the mussels to feed more efficiently in turbid water, which explains the observed differences in CR response to TSS. These results demonstrate the importance of multistressor approaches in evaluating aquatic organisms, as well as providing a potential mechanism to explain how mussels thrive in turbid rivers.

Increases in total suspended solids (TSS) in rivers have likely contributed to the decline in unionid mussel population sizes as feeding and reproduction are reduced at high TSS concentrations. Surprisingly, however, unionids are often found in turbid rivers. We predicted that clay‐sized particles, which comprise > 80% of river seston (particles in suspension), were too small to affect unionid clearance rate (CR) and thus explain this conundrum. We examined this hypothesis in laboratory experiments involving four unionid species exposed to four particle‐size based TSS treatments (mixed sediment: 0–63 μm; clay: 0–5 μm; fine silt: 5–38 μm; coarse silt: 38–63 μm; sourced from mussel sites) at 20 mg/L, which is a TSS concentration sufficient to reduce CR. Whereas the CR for all species was lower for mixed sediment, coarse silt and clay treatments—the latter was opposite to our prediction—the CR of mussels given the fine silt treatment was similar to the no‐TSS control for two species and higher than the other TSS treatments in the other two species. Fine silt contained the most fluorescent (i.e. algal) particles and the highest protein and lipid content, which suggests that CR were higher on the more nutritious diet. Particle quality, rather than size, is what modulates suspension feeding in turbid rivers. Our current understanding of the ecological effects of bivalve suspension feeding will need to be revised to incorporate field‐based measurements.

Unionid mussels are among the most imperilled freshwater taxa globally (Lopes-Lima et al., 2017). Their populations have been threatened by factors including over-exploitation (e.g. Strayer et al., 2004), invasive species (e.g. Ferreira-Rodríguez, Sousa, &