Shape: Elongate, with a broadly curved dorsal margin and slightly concave ventral margin.
Periostracum: Light brown (juveniles) to dark brown or black. No shell rays, but growth lines are prominent and heavy.
Lateral Teeth: One poorly defined lateral tooth on each valve, though these are sometimes hard to distinguish.
Pseudocardinal Teeth: Right valve has one triangular-shaped tooth that is slightly down-turned. Left valve has two triangular-shaped teeth; the posterior tooth is larger with a ragged edge, and the anterior tooth is smaller and sometimes indistinguishable.
Nacre: Usually purple, salmon-colored, or pink (sometimes white). Nacre color fades to white over time. Anterior adductor muscle scar is sharply defined, whereas the posterior adductor muscle scar is less defined. Tiny faint pits are sometimes evident on the central part of the nacre.
This species is one of four North American species that may be hermaphroditic, meaning that individuals may have both male and female reproductive traits. However, this condition is rare in M. falcata populations, and most populations have separate sexes. Precise timing of breeding and release of glochidia is not known. Fertilization is thought to occur in the spring and gravid females may be found from late summer to early spring. In M. margaritifera, the timing of glochidia release and the amount of time spent attached to host fish are strongly influenced by temperature. Glochidia of M. falcata were released from mid-June to early July in the Truckee River of California, when water temperatures increased from 10 degrees to 15 degrees Celsius. For the closely related M. margaritifera, release of glochidia may occur over a period of several months beginning in late summer, and glochidia may overwinter on the gills of host fish. Host fish for M. falcata are thought to include native and non-native trout and salmon, including cutthroat trout, rainbow trout, Chinook salmon, coho salmon, redband trout, sockeye salmon, steelhead trout, brook trout, and brown trout. Non-salmonid hosts have been suggested based on limited laboratory tests. Average life spans of Margaritifera are sixty to seventy years, with some living more than one hundred years, making them among longest-lived animal species on Earth.
M. falcata seem to prefer cold clean creeks and rivers that support salmonid populations. They can inhabit headwater streams less than a few feet wide, but are more common in larger rivers. Less commonly, this species can be found in more degraded habitats such as irrigation ditches in Washington and Oregon. Sand, gravel, and cobble are preferred substrates, especially in stable areas of the streambed. Large boulders help create these stable environments by anchoring the substrate and creating a refuge from strong currents. Banks and pools are often favorable habitats because the currents are weaker, shear stress is lower, and the substrates are more stable M. falcata does not tolerate sedimentation. In Idaho’s Salmon River, M. falcata covered with a substantial amount of sand and gravel were unable to move to the surface and perished. In environments where host fish are abundant, physical habitat is ideal, and human threats are minimal, <>M. falcata can attain very high densities (>300 per square yard), often carpeting the stream bottom. In 1981, Clarke wrote, “In favourable localities in British Columbia the mussels may be so abundant and closely packed that they completely obscure the stream bottom."
M. falcata is found in Pacific drainages from California to British Columbia and southern Alaska. Some scientists consider some of the coastal and large-river populations extirpated, nearly extirpated, or declining rapidly. This species is still common throughout parts of the northern Rockies, although some populations in Montana may be declining. It is also found east of the Continental Divide in the headwaters of the Missouri River. Originally, these populations were thought to be the eastern species M. margaritifera but recently scientists have confirmed that the populations are M. falcata and that the species crossed the divide. The most likely explanation for this distribution is headwater capture, where preglacial watersheds were cut and reconfigured by glacial advance or retreat. West-slope cutthroat trout are thought to have crossed the Continental Divide from the West into the headwaters of the present-day Missouri River during the Pleistocene glaciation, more than 20,000 years ago. Since cutthroat trout are an important host for M. falcata, it is likely that mussels hitched a ride on the trout.
Distribution in British Columbia
This species is found across southern BC (reported as far east as Jaffray) and north to Babine Lake (Gelling 2008).
Recent conservation concerns about M. falcata closely mirror well-known stories of the decline of Pacific salmon fisheries. Both need clean cold streams and rivers, and M. falcata reproduction requires salmon and trout hosts. The greatest threats to western pearlshells come from loss of host fish species and water diversion projects for irrigation, power generation, and water supply, particularly in Washington, Oregon, Idaho, and California. Dams destroy many miles of free-flowing rivers, disrupt native fish communities, and may have contributed to the demise of many populations of western pearlshells. Agriculture and rapid urbanization are affecting aquatic ecosystems throughout the West through nutrient enrichment, siltation, and chemical pollution, all of which may negatively impact western pearlshells. Climate change has been implicated in the decline of the closely related M. margaritifera in North America and Europe and it is likely that M. falcata will be affected in similar ways.
Invasive species that compete with native fish may affect M. falcata. In some locations where western pearlshells are still abundant, native cutthroat trout are being replaced by nonnative rainbow, brown, and brook trout. The long-term effects of increasing nonnative fish populations on native mussels, albeit with fish species that may also serve as hosts, are unknown. Native hosts with which mussels have coevolved might be superior to nonnative hosts because their populations may be more stable in the long-term. Also, the mussel may be specifically adapted to traits unique to its native host, such as habitat use, behavior, and lack of immune responses to glochidial parasitism.
M. falcata has been extirpated throughout much of the mainstem Snake River and Columbia River of Oregon and Washington and has dramatically declined in abundance in one area of the Truckee River of California. This species historically existed in northern Utah, but has probably been extirpated from the state. The range of M. falcata is also contracting in Montana; historical populations from some larger rivers such as the Blackfoot, Big Hole, Bitterroot, and Clark Fork have been extirpated from the entire drainage, or are only present in low numbers. Many historic sites have been lost and some populations show little evidence of recruitment. The fate of this species throughout much of its native range remains uncertain.
“Margaritiferidae is a family of medium-sized freshwater mussels... known as freshwater pearl mussels, because they are capable of producing pearls.” Wikipedia 2009
Recommended citation: Author, Date. Page title. In Klinkenberg, Brian. (Editor) 2017. E-Fauna BC:
Electronic Atlas of the Fauna of British Columbia [efauna.bc.ca]. Lab
for Advanced Spatial Analysis, Department of Geography, University of British
Columbia, Vancouver. [Accessed:
20/07/2017 5:32:20 AM]
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