Freshwater unionid mussels (Mollusca, Bivalvia, Unionidae) are one of the most highly threatened and rapidly declining groups of freshwater organisms. They are sensitive to water quality conditions, and the presence of some species is considered an indicator of water quality. In addition, their life history traits (e.g. relatively immobile, long lifespan, filter-feeding, parasitic larval life stage on fish) make them susceptible to habitat disturbance and instream flow alterations in particular, since they cannot quickly adjust to flow fluctuations, or other regulations in instream flows. Because of their parasitic larval stage, they are also susceptible to habitat alterations affecting their host species.
Freshwater mussels appear to have specific habitat requirements, such as water depth and water velocity, similar to freshwater fish, and thus their suitability criteria can be considered during instream flows studies described above. However, their inability to react quickly to changes in flows complicates the procedures used to assess effects on other aquatic species. The following reference provides additional detail on an approach to evaluating the effects of hydroelectric project on freshwater mussels:
Layzer, J. B., and L. M. Madison. 1995. Microhabitat use by freshwater mussels and recommendations for determining their instream flow needs. Regulated Rivers: Research and Management 10: 329-345.
