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Freshwater mussels

Freshwater mussels

King County, Washington

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 dark 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.

Margaritifera falcata

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.

The Locals

Although one would expect to see mussels in most of our streams and rivers, they often aren't there. They are only found in the cleanest streams and rivers with cool, clear water and bottoms that aren't muddy. Western pearlshell (Margaritifera falcata) live in rivers and streams with cool, flowing water.

Adults prefer sand/gravel substrates. The juveniles prefer well-oxygenated sand, which is usually found behind debris jams or at the base of woody debris. The most common mussel in our area is M. falcata with observations in an area of northern Big Bear Creek of up to 150 mussels per half square meter (Rensel 1992). An identification tip: most Margaritifera falcata have purple color on the inside of their shell and other local mussels do not.

Oregon floaters (Anodonta oregonensis) are found in slower moving water. They are commonly found in mud, sand, or fine gravel beds. The juveniles attach to gravel in well aerated, flowing waters but the attachment threads dissolve as they age. When this happens, the mussels are washed downstream where they settle to the sandy bottom in slower moving water. The Anodonta has also been seen in our area.

Western ridgemussel (Gonidea angulata) are found in rivers and only in Pacific drainages (Toy 1998).

Identifying these Pacific Northwest Natives

Click here to download the Freshwater mussels identification card

mussel ID card

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 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.

Zebra mussels are 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.

King County Reports

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.

Additional Info

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

The Pacific Northwest Native Freshwater Mussel Workgroup is a local group focused on relevant mussel research, management, and educational activities.

This thesis was cited above: Toy, K.A. 1998. Growth, reproduction and habitat preference of the freshwater mussel Margaritifera falcata in Western Washington. Thesis. University of Washington. Seattle, Washington.