Invertebrates

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 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.

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 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).

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).

Life in the mud

Adult freshwater mussels don't lead a very exciting life. They stay in one spot, bury their back end in the soft river bottom, and leave their front end and two siphons exposed like snorkels. They continuously pump water through their bodies, in one siphon and out the other. They use their gills to filter oxygen and food from the water. Their food consists of plankton (microscopic plants and animals) and other organic matter suspended in the water. And as they eat, they are cleaning the water.

Reproduction

In the summer when the mussels are ready to reproduce, the males release sperm into the water and the females catch what they can. The sperm is siphoned by the female and used to fertilize her eggs internally. If they aren't grouped together, reproduction is hard to achieve. After fertilization, the female then holds up to several thousand eggs at a time in her gills. There they can get oxygen and have a place to brood until they develop into glochidia—the larval stage of mussels.

Then in the late spring or early summer, the glochidia are expelled into the water where they have to fend for themselves. They must attach themselves to the gills of a host fish within a couple days. Most freshwater mussels team up with only one type of fish. Our northwest mussel species favor salmon, so without salmon, mussels cannot successfully reproduce.

Once the larval mussels attach to the fish, the fish body reacts to form a cyst that covers them with cells. The glochidia remain in the cysts for two to five weeks (depending on the temperature). Hitchhiking on a fish is a baby mussel's only opportunity to travel, which results in a free ride to a new home.

After the mussels change from the larval form and begin to resemble adults, they break out of the cyst and fall to the bottom of the stream. They bury themselves in the bottom and begin to live an independent life. Only one in a million survive to the adult stage, but to offset these low odds, mussels lead a very long reproductive life and produce millions of eggs per year!

Mussel loss

Because mussels have to filter the water wherever 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 their river habitat. Mussels need the same clean, cool, oxygenated water that our salmon need and they also 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.
4. Habitat loss: Channelizing, dredging and otherwise altering streams and buffer zones threaten, and may even remove, the homes of mussels.