The western pearlshell mussel (Margaritifera falcata) has the broadest distribution and longest lifespan of the western U.S. freshwater mussel species. It’s found in small streams and large rivers that support salmonid populations from California to Alaska and inland to western Montana, Wyoming and Utah. Individuals have been recorded to live in excess of 100 years old and reach sizes up to about six inches long. Although die-offs and substantial declines have been reported in some drainages, in other locations this species can still be found in large, densely populated aggregations consisting of thousands of individuals. Mussels can greatly affect stream biodiversity by filtering water, recycling nutrients, providing food and habitat for aquatic invertebrates, and serving as an important food source for terrestrial and aquatic mammals.
The western pearlshell inhabits perennial, cold-water streams and rivers throughout its range. Since western pearlshell young (glochidia) require a salmon or trout species as a host fish to survive, they are limited to salmonid-bearing streams or rivers (either resident or anadromous forms of salmonid fishes can be hosts for the western pearlshell mussel). Western pearlshell mussels can be found over a range of habitat types but are more common and abundant in low-gradient stream reaches with stable sand, gravel, or cobble stream bottoms. In smaller streams, western pearlshell aggregations can be found anywhere from undercut banks to the middle of the stream with the highest flow (the thalweg). In larger rivers, and streams with higher gradient, western pearlshell mussels can be found in areas that provide shelter from high stream flows and streambed scour such as behind boulders or log jams.
A natural body of running water.
Like all freshwater mussels, western pearlshell are filter feeders, siphoning suspended particles from the water column. They may consume plankton, bacteria, dissolved organic matter, or algae. This filtering provides an important water quality service by reducing turbidity and controlling nutrient levels, especially where there are dense mussel beds. Particles not used by the mussel are often re-formed and expelled as larger particles that are in turn used as food by other aquatic life.
Freshwater mussels are sedentary organisms, t spending their entire lives near where they settled. They can use their foot to move laterally across sediments (usually short distances) or vertically within the sediments. Mussel movements may be related to reproduction or a response to physical disturbance and environmental changes such as water temperature, stream flows, or scouring of the stream bed. Mussel displacement due to high flows or dislodgement due to predators or human activity also likely occurs. The main mode of mussel dispersal comes through a stage of development when larvae (glochidia) parasitize a host fish and move with the host within the aquatic system.
Western pearlshell are brown to black and reach sizes up to about six inches long. They are elongate and slightly concave on the ventral margin with a curved dorsal margin. Erosion marks can be prominent on the umbo region. They have one pseudocardinal tooth on the right valve and two on the left valve, however one is often poorly defined. They also have one lateral tooth on each valve; however, they are hard to distinguish. The inside of the shell can be purple to pinkish and sometimes white.
The lifecycle of the western pearlshell, and most freshwater mussels, requires a host fish for their parasitic larvae (glochidia) to attach to. The known host fish for the western pearlshell are salmonids, including Chinook salmon (Oncorhynchus tshawytscha), coho salmon (Oncorhynchus kisutch), cutthroat trout (Oncorhynchus clarkii), redband trout (Oncorhynchus mykiss), and steelhead (Oncorhynchus mykiss). The glochidia develop after the females eggs are fertilized by filtering in water containing sperm released from mature males, although there is genetic evidence that suggest that they can be hermaphroditic. The glochidia are released from the females in aggregates bounded by mucus (termed conglutinates) and eventually will attach to the gills of nearby salmonids. They become encysted and can stay attached for weeks to months before releasing from the fish as juvenile mussels. Once released, they settle down in the stream bottom and burrow in. Some individuals have been documented to reach over 100 years old.