Eric Snyder, Ph.D.
In North America, it is estimated that 72% of the nearly 300 species of freshwater mussels (unionids) are either extinct or in danger of extinction and only 23% are considered stable. No other group of aquatic animals in North America is in such grave danger. They are one of the most endangered groups of animals on Earth, yet surprisingly little is known about their life history, habitat needs, or even how to distinguish different species. Our project investigated the status of the unionid community in a 3rd order Michigan stream and examined their relationship with aquatic macroinvertebrates.
We sampled randomly selected transects within two separate 100 meter reaches that spanned natural gradient in mussel densities in Cedar Creek to compare unionid and macroinvertebrate biodiversity, density, and species richness. At each site, we sampled macroinvertebrates, unionid mussels, and environmental variables such as visually estimating the percentage of substrate composition. At each site, a Hess Net was used to sample benthic macroinvertebrates, and eight, 0.25m2 quadrats were surveyed for unionids in the area directly surrounding the Hess Net (2m2 per site total)(n=144). We also recorded other environmental variables such as water temperature, velocity, total dissolved solids (TDS), concentration and rate of organic matter (OM) transport at the top of each reach, pH, and specific conductivity. A total of seven freshwater mussel taxa were found including the spike (Elliptio dilatatus), plain pocketbook (Lampsillis cardium), fatmucket (Lampsillis siliquoidea), wabush pigtoe (Fusconaia flava), rainbow (Villosa iris), and the federally endangered clubshell (Pleurobema clava) and rayed bean (Villosa fabalis). Mussel density was greater in areas of faster water velocity (p=0.002). Transported OM input into the reach with high mussel density was significantly higher (p=0.0007) vs. the reach with low mussel density (average of 0.076 vs. 0.046 g/m3/sec, respectively). Mussel density was positively correlated with macroinvertebrate density across the randomly chosen 18 sites (p=0.004, R2=0.405). High variation in substrate size and composition was positively correlated with high mussel density vs. more homogeneous in areas with few or no mussels. The data suggest that substrate is a strong determinant of mussel assemblages and as the natural gradient in substrate becomes more homogeneous, mussel abundance and diversity declined. As other research has shown, we found a positive relationship between mussels and aquatic macroinvertebrates. Future research at this site could investigate the mechanisms leading to this association and follow up on our finding that higher mussel density was positively correlated with high variation in substrate, faster water velocity, and higher concentrations transported OM—a likely food resource for both mussels and filter-feeding macroinvertebrates.
*This scholar and faculty mentor have requested that only an abstract be published.