Suspension Feeding of Juvenile and Adult Freshwater Mussels (Bivalvia: Unionidae) Under Flowing Conditions

I examined the feeding abilities of adult and juvenile freshwater unionid mussels, using two closely related species, Lampsilis siliquoidea and Lampsilis fasciola, and two other species within the Lampsilinae subfamily, Ligumia nasuta and Villosa iris. My experiments were designed to determine how algal/particle flux influences the ability of mussels to remove suspended material (clearance rate, CR) using recirculating flow chamber systems. Juveniles were exposed to algal flux (Chlorella vulgaris), under ecologically relevant velocities. The CRs of L. siliquoidea (1 – 4 week old), L. fasciola (1 – 3 week old), L. nasuta (1 week old) and V. iris (2 week old) increased with algal flux across all age groups examined, and there was some indication of nonlinearity as very high flux. Adults were exposed to river seston flux, and their ability to remove suspended material was measured based on particle size and quality. The CR of all species increased linearly with chamber velocity, but the relationship was non-linear for CR vs. flux indicating saturation of CR at high flux. Flow cytometry on the Lampsilis species indicated that both adult species had higher CR for larger particles (>10 μm) than smaller particles. Adult L. fasciola had high CRs for three algal species under low flux (a centric diatom species, a pennate diatom species, and Chlorella) however they were unable to discriminate among algal species with increased flux. The CR of L. siliquoidea increased with increasing flux for centric diatom species but decreased with flux for the Chloromonas species. The results of this study provide some intriguing evidence for niche separation among four different unionid species, facilitated in part by hydrodynamics, which may help our understanding of their habitat requirements.