|Scientific Name:||Margaritifera falcata (Gould, 1850)|
Alasmodon falcata Gould, 1850
Alasmodon yubaensis Trask, 1855
|Taxonomic Source(s):||Gould, A.A. 1850. Shells from the United States Exploring Expedition. Proceedings of the Boston Society of Natural History 3: 292-296.|
|Taxonomic Notes:||Gould (1850) originally described the species from “Wallawalla, Oregon; Sacramento River, California” and placed the species under the genus Alasmodon (Alasmodonta). Trask (1855) described Alasmodon yubaensis from the Yuba River in California. Lea (1856) synonymized both Alasmodon falcata and Alasmodon yubaensis under Margaritana margaritifera. The species has variously been assigned to different genera, including both Margaritana (Schumacher, 1817) and Margaritanopsis (Haas, 1910); Henderson (1935) recognized the genus Margaritifera, listing the species as Margaritifera margaritifera falcata. Heard and Guckert (1970) and Turgeon, et al. (1998) have since established Margaritifera falcata at the species level. A list of synonyms can be found at the Mussel Project Web Site: MUSSELp (Graf and Cummings 2015).|
|Red List Category & Criteria:||Near Threatened ver 3.1|
|Assessor(s):||Blevins, E., Jepsen, S., Brim Box, J. & Nez, D.|
|Reviewer(s):||Tait, C. & Mageroy, J.|
|Contributor(s):||Mageroy, J., Fallon, C., Hatfield, R., Howard, J., LaBar, C., Maine, A., Mock, K., O’Brien, C., Smith, A., Tait, C. & Zarnoch, J.|
We used a dataset of western freshwater mussel occurrence records (shells and/or live animals) from nearly 300 sources spanning the 1830s to 2015 (Xerces/CTUIR 2015; full list of contributors at: http://www.xerces.org/western-freshwater-mussel-database-contributors/). This analysis examined change in extent of occurrence (EOO) and area of watershed occupancy from occurrence (presence) data prior to 1990, and from 1990–2015. Based on 646 records dating prior to 1990 and 1,887 records dating 1990–2015, Margaritifera falcata has declined in EOO by 1% (16,816 km²) but in watershed area by 17% (86,039 km²). Decline in watershed area is reinforced by more regional and local abundance observations, especially observations of decline that have been reported since 1990, which are not reflected in the analysis. Recent work by Stagliano (2015) reports that, of 85 populations in Montana, only approximately 20 are considered viable in the next 100 years; a statewide reduction in range was also reported. Thus, future decline is also anticipated. Mock et al. (2013) also report high levels of localised inbreeding range-wide and suggest that possible causes include low dispersal rates and/or high rates of hermaphroditism and selfing due to low population densities.
A number of extant populations in Montana are also currently considered non-viable; indeed, twenty-five percent of surveyed streams where a mussel population was considered non-viable in 2010 are already extirpated (Stagliano 2015). Hovingh (2004) discusses the decline of the species in Utah, implicating loss of native host fishes as a major factor, and recent surveys by Richards (2015) in the Jordan River did not detect the species, despite the species’ historical presence in the Jordan drainage and recent intensive survey effort. Similarly, populations in the interior of the species range may also have been more recently extirpated, such as in the Umatilla River of Oregon (Brim Box et al. 2006). Other declines and extirpations have been reported in Oregon, Idaho, and Washington (reviewed in Jepsen et al. 2012), including an estimated decline in extent between 37 and 57% in Idaho (Lysne and Krouse 2011).
Although this species may be locally abundant throughout the range, there have been reports of declines in distribution, population size and/or observation of populations without apparent recruitment in California, Idaho, Montana, Nevada, Oregon, Utah, Washington, and Wyoming (reviewed in Jepsen et al. 2012, Furnish 2010, Southern California Edison Company 2010, Lysne and Krouse 2011, Howard et al. 2015, Stagliano 2015, May and Pryor 2016). Howard et al. (2015) found that only 59% of 22 revisited historical sites in California were still occupied by this species. One population numbering as many as 20,000 individuals in 1941, one of the few estimates available for historic population size, was revisited in 2006 and has since declined to ~120 individuals in a 2 km stretch at one site in the Truckee River of California (Murphy 1942, Western Mollusk Sciences 2008, Howard et al. 2015). A paucity of abundance data limits accurate assessment of decline in this species when populations decrease but still persist in low numbers. Still, populations in British Columbia appear to be secure (NatureServe 2015), and recent surveys in Wyoming suggest the species is stable there, with numerous individuals and evidence of juvenile recruitment both observed (Mathias and Edwards 2014). The status of the species in Alaska and Nevada is not well understood, although observations have been few (Jepsen et al. 2012, Smith et al. 2005).
Margaritifera falcata is a long-lived species (maximum lifespan greater than 100 years; Toy 1998), and populations may be in decline for many years before becoming extirpated, particularly in populations without recent recruitment. This phenomenon has been documented in Margaritifera margaritifera in Britain and Europe (Mollusc Specialist Group, 1996). The Near Threatened category applies to species that do not yet meet the qualifying criteria for the Vulnerable category, but that may be close to qualifying or are likely to become threatened if existing conservation actions diminish or cease (IUCN 2016). Evidence that reduced population size often precedes population extirpation suggests that the declines in EOO and watershed area underestimate decline in population size for this species. Based on reports of more recent population extirpations in parts of the range and observations of large declines in population abundance, a reduction in watershed area of 17% indicates a greater decline in population size, thus approaching the 30% threshold for Vulnerable. Therefore, the species is currently considered Near Threatened.
|Range Description:||Margaritifera falcata, the only member of the family Margaritiferidae in western North America, is broadly distributed from the Kern basin in California, where it was also historically found, north to central British Columbia and the far southern coast of Alaska. It is also found in Oregon, Washington, Idaho, western Montana and Wyoming, and northern Utah and Nevada. One spurious record previously reported for the Bill Williams River in Arizona in 1993 (WCMAFE/BLM/USUNAMC, 2015) has since been corrected to “Unionoida”, as Margaritifera falcata is not known to occur in the state (Myers, pers. comm. 2015). Indeed, although R.E. Call (1884) reported the species “south to Arizona”, Walker (1916) considered any occurrence in Arizona as “very doubtful” and no other records support the species’ occurrence there. Myers (2009) has discussed the presence of Margaritifera shells in the Colorado River basin, concluding that the shells have been transported from outside the basin. The geographic range of the species is poorly understood in Alaska, where multiple records have been reported from the Alaskan panhandle, but a single specimen was also reported from the Deshka River, 1,000 km northwest of the northernmost panhandle records (Gelvin-Reymiller, pers. comm. 2009). Similarly, the status of populations in Nevada is poorly understood; Hovingh (2004) reported no observations of the species in Nevada, though there are now records of the species as recent as 2012 from three sites in northern Nevada. Nearly three times as many observations of this species have been reported since 1990 compared to pre-1990.|
This species is the only native western freshwater mussel with populations reported in southern California between 1990 and 2015, despite recent surveys in many historical sites for this and other species. At least one of three reported populations in the Kern River Basin was noted as having multiple age classes (Palenscar, pers. comm. 2015); however, other recent surveys of two historical sites in the Kern Basin found no extant populations (Hovingh 2004, Furnish 2010, Howard et al. 2015).
Native:Canada (British Columbia); United States (Alaska, California, Idaho, Montana, Nevada, Oregon, Utah, Washington, Wyoming)
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||Where present, M. falcata may occur in dense beds containing as many as 20,000 individuals (Truckee River, California; Murphy 1942). In the Siletz River on the Oregon Coast, Karna and Milleman (1978) reported a colony of approximately 100,000 M. falcata near River mile 21, with densities exceeding 400 individuals/m². Other researchers have observed a maximum of ~230 individuals/m² (North Fork John Day River, Oregon; Brim Box et al. 2006) or much smaller aggregations (50-100 individuals in the South Fork Eel River, California; Howard and Cuffey 2003). Stagliano (2015) reports mussel aggregations containing one to generally no more than 50 individuals in Montana. Allard et al. (2015) also report between 0 and 55 individuals/m², with an average density of 9.5 individuals/m² in Merrill Creek, Oregon.|
Although this species may be locally abundant throughout the range, there have been reports of declines in distribution, population size and/or observation of populations without apparent recruitment in California, Idaho, Montana, Nevada, Oregon, Utah, Washington, and Wyoming (reviewed in Jepsen et al. 2012, Furnish 2010, Southern California Edison Company 2010, Lysne and Krouse 2011, Howard et al. 2015, Stagliano 2015, May and Pryor 2016). Howard et al. (2015) found that only 59% of 22 revisited historical sites in California were still occupied by this species. One population numbering as many as 20,000 individuals in 1941, one of the few estimates available for historic population size, was revisited in 2006 and has since declined to ~120 individuals in a 2 km stretch at one site in the Truckee River of California (Murphy 1942, Western Mollusk Sciences 2008, Howard et al. 2015).
Decline in watershed area is reinforced by more regional and local abundance observations, especially observations of decline that have been reported since 1990, which are not reflected in the analysis. Recent work by Stagliano (2015) reports that, of 85 populations in Montana, only approximately 20 are considered viable in the next 100 years; a statewide reduction in range was also reported.
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||Margaritifera falcata inhabits perennial rivers, streams and creeks 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 (summarized in Roscoe and Redelings 1964). This species prefers clear, cold water (Frest and Johannes 1995), and has been found at multiple elevations, including waterways above 5,000 feet (Smith, pers. comm. 2010) and even 8,000 feet (Mathias, pers. comm. 2016). The species occurs in waterways with low velocities, low shear stress and stable substrates (Howard and Cuffey 2003, Vannote and Minshall 1982, Stone et al. 2004, Davis et al. 2013). Margaritifera falcata is frequently found in eddies or pools (Howard and Cuffey 2003) and areas with stones or boulders that likely shelter mussel beds from scour during flood events (Vannote and Minshall 1982); indeed, mussel beds appear to occupy the most stable areas of the river bed (May and Pryor 2015). This species appears to be intolerant of sedimentation; in the Salmon River of Idaho, mussels that were covered with sand and gravel were unable to uncover themselves and ultimately perished (Vannote and Minshall 1982).|
Like other margaritiferids, M. falcata requires a host fish to complete development. Glochidia (the larval stage) are released from April through July in northern California (Murphy 1942, Spring Rivers 2007) and March to June in Oregon (Meyers and Milleman 1977, Karna and Milleman 1978, O’Brien et al. 2013). Glochidia are released as a white conglutinate similar in shape to mussel brood chambers or gills. Several species of host fish have been confirmed (i.e., glochidial infestation of the fish has been observed in the wild and metamorphosis of the glochidia has also been observed) or are considered potential hosts (either glochidia infestation has been observed only under artificial conditions or glochidial metamorphosis has not been observed; O’Brien and Brim Box 1999, O’Brien and Williams 2002, O’Brien et al. 2013). Confirmed native and nonnative host fish include Coho Salmon (Oncorhynchus kisutch), Chinook Salmon (Oncorhynchus tshawytscha), Cutthroat Trout (Oncorhynchus clarkii), Rainbow Trout/Steelhead Trout (Oncorhynchus mykiss), and Brown Trout (Salmo trutta) (Murphy 1942, Karna and Milleman 1978). Potential native and non-native host fish include Bull Trout (Salvelinus confluentus), Kokanee Salmon (Oncorhynchus nerka kennerlyi), Lahontan Redside (Richardsonius egregius), Prickly Sculpin (Cottus asper), Speckled Dace (Rhinichthys osculus robustus), Tahoe Sucker (Catostomus tahoensis), Threespine Stickleback (Gasterosteus aculeatus), and Brook Trout (Salvelinus fontinalis) (Murphy 1942, Meyers and Milleman 1977, Karna and Milleman 1978, Steg unpub. obs. cited in Jepsen et al. 2012).
|Continuing decline in area, extent and/or quality of habitat:||Unknown|
|Generation Length (years):||9-45|
|Use and Trade:||Freshwater mussels have cultural significance for First Nations and Tribes in western North America, including as a traditional food resource (COSEWIC 2003, CTUIR 2015).|
Threats to Margaritifera falcata and other species of freshwater mussels in North America include: impoundments and loss or decline of host fish, channel modification from channelization, dredging and mining, restoration activities that impact streambanks or streambeds or dewater channels, contamination, sedimentation and scouring, nutrient enrichment, water withdrawal and diversion, thermal pollution, livestock grazing in riparian areas, shoreline development, and the introduction of non-native plant, fish and invertebrate species (Jepsen et al. 2012). Many of these impacts, especially a reduction in stream flow and thermal pollution in arid areas, are being exacerbated by climate change. Additionally, the species may be threatened by low genetic diversity as a result of recent population reductions, although it is difficult to detect the molecular signature of a genetic bottleneck for species with hermaphroditic life histories (Mock et al. 2013). Multiple salmonid host fish are listed as endangered or threatened in parts of their range under the U.S. Endangered Species Act. Declines in host species’ population size and distribution may have significant impacts on this species.
Few abundance estimates are available for existing populations, and anecdotal declines have been reported across the range, although causes of these declines are not well understood. Additional surveys to document the viability of extant populations throughout the range and monitoring of populations considered non-viable should be prioritised, as should protection from and remediation of known threats.
|Errata reason:||This errata version of the assessment includes the assessment contributors, the distribution map (both were missing from the original assessment), and addresses some minor formatting issues.|
|Citation:||Blevins, E., Jepsen, S., Brim Box, J. & Nez, D. 2016. Margaritifera falcata (errata version published in 2017). The IUCN Red List of Threatened Species 2016: e.T91109639A114128748.Downloaded on 19 April 2018.|
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