Western pearlshell mussel (Margaritifera falcata)

Physical description

Western pearlshell is a bivalved freshwater mollusk. They have an oblong shape, and they are up to 5 inches long. Their shell may be light brown, dark brown or black. The shell is curved on the top and somewhat concave on the bottom along the ventral margin. For more details (including photos), see the Xerces Society’s guide to Freshwater Mussels of the Pacific Northwest.

Ecology and life history

Freshwater mussels are found in shallow habitats in permanent bodies of water, including creeks, rivers, and ponds generally at low elevations. Mussels tend to concentrate in areas of streams with consistent flows and stable substrate conditions. They are often absent or sparse in high-gradient, rocky rivers, but are frequently encountered in low-gradient creeks and rivers, perhaps because they provide a variety of habitat conditions, reliable flow, good water quality, and diverse fish communities. The mussels are filter feeders that consume phytoplankton and zooplankton suspended in the water.

Western pearlshell mussels inhabit cold creeks and rivers with clear, cold water and sea-run salmon or native trout including waterways above 5,000 feet in elevation. These mussels are typically found at depths of 1.5 to 5 feet, and they tend to congregate in areas with boulders and gravel substrate, with some sand, silt and clay. These mussels occur in waterways with low velocities and stable substrates and are frequently found in eddies or pools and areas with stones or boulders that likely shelter mussel beds from scour during flood events. This species appears to be intolerant of sedimentation.

Freshwater mussels have a complex life cycle. They have separate sexes, although hermaphrodites (individuals with male and female traits that are capable of self-fertilization) have been documented for some North American species, including the western pearlshell. During breeding, males release sperm into the water and females filter it from the water for fertilization to occur. Embryos develop into larvae called glochidia, which are released into the water and must encounter and attach to a fin or gill filaments of host fish. Glochidia form a cyst around themselves and remain on a host for several weeks. They subsequently release from the host fish and sink to the bottom, burrow in the sediment and remain buried until they mature.

Like other freshwater mussels, western pearlshell mussels rely on host fishes to reproduce and disperse. Because freshwater mussels are not able to move far on their own, their association with fish allows them to colonize new areas or repopulate areas from which they have been extirpated. During their lives, mussels may move less than a few yards from the spot where they first landed after dropping from their host fish. Documented host fishes for western pearlshells include cutthroat trout, rainbow/steelhead trout, Chinook salmon, and brown trout, and a number of other fish are considered potential hosts.

Freshwater mussels that live in dense beds, including western pearlshell and western ridged mussels, provide an important water purification service; they can filter suspended solids, nutrients and contaminants from the water column and collectively improve water quality by reducing turbidity and controlling nutrient levels.

Western pearlshell mussels have an average life span of approximately 60 to 70 years, although some individuals are thought to have lived more than 100 years. Because this species is sedentary, sensitive to environmental changes, and long-lived, it can be an excellent biological indicator of water quality.

Geographic range

The range of the western pearlshell mussel extends from Alaska and British Columbia south to California and east to Nevada, Wyoming, Utah and Montana; it is apparently most abundant in Oregon, Washington, Idaho and British Columbia.

In Washington, western pearlshells have been extirpated from much of the mainstem Columbia and Snake Rivers; substantial declines, die-offs, or lack of recent reproduction have also been reported from the SanPoil River (Ferry County), Kettle River (Stevens County), the Little Spokane River (Spokane County), Snohomish River, Muck Creek (Pierce County), Bear Creek (King County), and Nason Creek (Chelan County). High levels of arsenic and organochlorine pesticides were found in the tissues of other mussel species collected from the mid-Columbia River during that survey.

This species has also been extirpated from northern Nevada, from most areas in northern Utah, several rivers in Montana, and numerous other locations. In addition, there are reports of populations of western pearlshells that apparently have not reproduced for decades. Populations of such a long-lived species may appear stable, when in fact they are not reproducing; populations showing repeated reproduction, evidenced by multiple age classes, are now rare.

For a map of range-wide distribution and conservation status of this species, check out Nature Serve Explorer.