Freshwater mussels

When mussels are mentioned in the Pacific Northwest, many people probably think of the edible blue mussels attached to rocks and pilings and exposed at low tides. The Pacific Northwest is also home to freshwater mussels, which are in an entirely different family of mussels. They are black-brown in color and bury themselves in mud, sand, and rocks where they live in the bottoms of our streams.

Freshwater mussels are generally camouflaged in the bottom of the streams. We typically don't notice them, but where populations exist, they filter and clean the water. Their sensitivity to pollutants make them an indicator of stream health that can help us track sources of pollution.

Identifying these Pacific Northwest Natives

Click here to download the new Mussel ID card, produced by The Water Tenders. The file is an Adobe Acrobat pdf file.

Freshwater Mussels of the Pacific Northwest, an excellent booklet, was funded by the U.S. Fish and Wildlife Service, Water Tenders through a grant from the King County Water Quality Fund, and the Xerces Society for Invertebrate Conservation.

Mussel Diversity

The United States has the richest diversity of mussels in the world with about 300 different species. Unfortunately, it is a very endangered group of animals. It is believed that about 10% of the species are already extinct and it is estimated that that 70% are at risk of disappearing.

Most of the US species live in the southeast, which is home to the richest collection of mussels in the world. In western Washington, we have only three species: western pearlshell (Margaritifera falcata), Oregon floater (Anodonta oregonensis), and western ridgemussel (Gonidea angulata). The western pearlshell is pictured above.

Mussel Age

The surface of the shell has distinct black lines or ridges that represent winter rest periods. These ridges can be counted to estimate the age of a mussel, just as one counts the rings on a tree.

Mussels are long-lived—some species can live over 100 years. There have been individuals of our local species, western pearlshell (M. falcata), found to be as old as 85 years in Bear Creek (Toy 1998). The mussel pictured above is from Bear Creek and is about 50 years old.

Mussel Loss

Mussels are very sensitive to the quality of their river habitat and are often thought of as good indicators of the health of a stream. Because they have to filter the water where they land as larvae, they ingest whatever is around them. They can't choose what they eat and are therefore sensitive to toxins and pollutants in the stream. Mussels need the same clean, cool, oxygenated water that our salmon need and, actually, they need the salmon themselves to survive. Across their range, there are four primary threats to mussel survival:

1. Runoff from land development:

2. Water diversions for industrial, domestic, and agricultural uses:

3. Non-native invasions: Non-native zebra mussels are outcompeting the natives in the midwest and southwestern US; it is believed that it is merely a matter of time before zebra mussels appear in the Pacific Northwest—see below.

4. Habitat loss: Channelizing, dredging and otherwise altering streams and buffer zones threaten, and may even remove, the homes of mussels.

Zebra Mussels

The thumbnail-sized zebra mussel, Dreissena polymorpha, is invading America. Zebra mussels reproduce faster and more efficiently than our natives because their larvae are free swimming and do not need a host fish to grow to adult stage. They reproduce in large numbers, can live on almost any surface, and are therefore very successful at inhabiting US waters. There are no known natural enemies of the zebra mussel in the US at this time and once they are established, they are hard to eradicate or control.

Although not found in Pacific Northwest waters yet, there is great concern that if and when they arrive, they could attach to fish ladders, screens, and hatcheries. Their shells are very sharp and could injure the passing fish, they could build up and disrupt water flow, and ultimately, they could threaten salmon recovery efforts.

For more information, visit Washington Department of Fish and Wildlife's website about zebra mussels in Washington.

Reports relevant to King County

Results of a Pilot Freshwater Mussel Survey in King County, by Bob Brenner. Published 2005 from work done in three King County streams in 2004.

Freshwater Mussels found in Bear and Cottage Lake Creeks during Habitat Assessments in 2001, by Karen Fevold and Jennifer Vanderhoof, 2002.

Investigation of Western Pearlshell Mussel (Margaritifera falcata) Mortality in Bear Creek, King County, Washington: A Disease Ecology Approach, by Arden Thomas. From work in 2008-- an investigation of western pearlshell mussel mortality in Bear Creek, consisting of field surveys, a caged mussel relocation experiment, and lake toxicity screening.

Toy, K.A. 1998. Growth, reproduction and habitat preference of the freshwater mussel Margaritifera falcata in Western Washington. Thesis. University of Washington. Seattle, Washington.

Resources

Check these links for more web sites about freshwater mussels:

Pacific Northwest Native Freshwater Mussel Workgroup
Fun with Fresh Water Mussels For younger people by the University of Vermont
Pictures of many mussels world-wide including our local Margaritiferafera Illinois Natural History Survey

King County, Washington