SHELL DECAY RATES OF FRESHWATER MUSSELS: INSIGHTS FOR SPECIES CONSERVATION

Freshwater mussels are among the most endangered taxa in North America. Many species are becoming extirpated from local areas at an alarming rate from the impacts of dams, logging, mining, and most recently urbanization of watersheds. A major obstacle to conservation planning is Shell decay rates of freshwater mussels: Insights for species conservation to document precisely when a species has disappeared from an area. Unfortunately, very little empirical data have been gathered on this problem so it is difficult to know when shell remains of extirpated species will disappear. We examined decay rates of shells of the invasive Asian clam (Corbicula fluminea) as well as the native Rainbow clam (Villosa iris). Six study sites were established in the highly urbanized habitats of Turkey creek, Third creek, and Second creek with two sites on each creek. At each site, sewn mesh bags containing Corbicula fluminea and Villosa iris, respectively, were anchored to the bed of the creek with steel rebar spikes. Twenty Corbicula fluminea valves and five Villosa iris valves were sealed in separate bags at each of the six sites. In each of the studied creeks, temperature, pH, water velocity, and calcium measurements were obtained to identify mechanisms behind shell decay. The mussels were baked at 52 degrees C for 45 minutes to remove moisture before weighing. After approximately 30 days, all twelve mesh bags were removed from the six sites and the shells reweighed to 0.01 mg. This procedure was repeated over several months to calculate the relative weight loss over time for the valves.

We found that the shell decay rates for both species varied considerably, from <1% per year to over 10% per year. Half-life estimates from these rates indicate that some shells may last for several decades whereas in other environments they may last for just a few years. This range is very similar to estimates made from experiments in coal mining and dam-affected habitats. While much further work remains to be done, our data indicate that water velocity and calcium concentrations in the waters are the major factors influencing shell decay rates.