Western Pearlshell Mussel

Margaritifera falcata
The western pearlshell mussel can look like just another rock in refuge streambeds/USFWS Photo Marie Fernandez

Freshwater mussels have a unique lifecycle. Eggs incubate and hatch into microscopic mussel infants – called glochidia – which need to attach themselves to the gills or fins of a passing host fish.  In Willapa Bay tributaries, that’s apt to be Coho salmon, cutthroat trout or a related species.  The time the glochidia stay attached varies from several weeks to several months, and may be influenced by water temperatures. Once they drop from the fish, they’ll burrow into the gravel bed and sediment of the river or lake the fish has reached. 

Of the two types most commonly found here, the western pearlshell mussels have an average life span of 60 or 70 years, although some can reach 100. However, their populations are thought to be dropping rapidly. The genus Anodonta, commonly known as floaters, grows faster and become larger, but their thinner shells make them an easy food source for predators like muskrats and raccoons. The pearlshells grow to about five inches in length and can be mistaken for rocks in the streambed.

To the refuge staff, the mussels have a “canary in a coal mine” role, serving as a warning of changes in environmental conditions. One of the staff’s major activities is to monitor habitats and populations of wildlife in the area. Decreases in mussel numbers can indicate problems for other species as well. Some species are already extinct and freshwater mussels are considered to be the most endangered group of animals in North America. 

To check on the mussel population, refuge biologists walk the streams, sometimes using a long tube viewer called an aquascope that allows them to see through the clear waters to the sediment where mussels live.  

The major problems influencing the local mussel population, refuge staff members say, are the same as those that affect the salmon population: water clarity and general stream quality. Freshwater mussels are vulnerable to water level fluctuation and can be adversely impacted by sedimentation. Both mussels and fish need clean, cold streams and rivers, biologists say.

Recently-retired refuge biologist Marie Fernandez conducted a pilot project five years ago to transfer 100 pearlshell mussels from large mussel beds on Bear River to three suitable streams within the refuge where their survival could be monitored. Each mussel was weighed, measured, photographed and marked with a numbered tag.  The locations of their new “homes” were noted so that they could be observed in later surveys. Once burrowed in the streambed, mussels seldom move more than a few yards in their lifetimes, unless dislodged and carried downstream by high water flows. 

After the initial project in 2007, more mussels were transferred from off-refuge sites annually through 2010. According to Fernandez, the new transplants were shown to be spawning in 2008 and again in a recent 2012 survey. More surveys are needed to determine whether the resulting mussel glochidia were successfully finding hosts and new homes.

Facts About Western Pearlshell Mussel

Need a fish host as part of their lifecycle

Can live up to 100 years of age

Look like rocks in streambeds