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Margaritifera falcata 

Scope: Global
Language: English
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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Mollusca Bivalvia Unionoida Margaritiferidae

Scientific Name: Margaritifera falcata
Species Authority: (Gould, 1850)
Common Name(s):
English Western Pearlshell
Synonym(s):
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 & 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 & Cummings, 2015).

Assessment Information [top]

Red List Category & Criteria: Near Threatened ver 3.1
Year Published: 2016
Date Assessed: 2016-03-14
Assessor(s): Blevins, E., Jepsen, S., Brim Box, J. & Nez, D.
Reviewer(s): Tait, C. & Mageroy, J.
Justification:

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). 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 nonviable; indeed, twenty-five percent of surveyed streams where a mussel population was considered nonviable 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 & 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 & Krouse, 2011; Howard, et al., 2015; Stagliano, 2015; May & 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 & 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.

Geographic Range [top]

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, personal communication, 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, personal communication, 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, personal communication, 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). 

Countries occurrence:
Native:
Canada (British Columbia); United States (Alaska, California, Idaho, Montana, Nevada, Oregon, Utah, Washington, Wyoming)
Additional data:
Continuing decline in area of occupancy (AOO):Unknown
Extreme fluctuations in area of occupancy (AOO):UnknownEstimated extent of occurrence (EOO) - km2:2643316
Continuing decline in extent of occurrence (EOO):YesExtreme fluctuations in extent of occurrence (EOO):Unknown
Continuing decline in number of locations:Unknown
Extreme fluctuations in the number of locations:Unknown
Range Map:Click here to open the map viewer and explore range.

Population [top]

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 & 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 & Cuffey, 2003). Stagliano (2015) reports mussel aggregations containing 1 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 & Krouse, 2011; Howard, et al., 2015; Stagliano, 2015; May & 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
Additional data:
Continuing decline of mature individuals:Unknown
Extreme fluctuations:UnknownPopulation severely fragmented:No
Continuing decline in subpopulations:No
All individuals in one subpopulation:No

Habitat and Ecology [top]

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 & Redelings, 1964). This species prefers clear, cold water (Frest & Johannes, 1995), and has been found at multiple elevations, including waterways above 5,000 feet (Smith, personal communication, 2010) and even 8,000 feet (Mathias, personal communication, 2016). The species occurs in waterways with low velocities, low shear stress and stable substrates (Howard & Cuffey, 2003; Vannote & Minshall, 1982; Stone, et al., 2004; Davis, et al., 2013). Margaritifera falcata is frequently found in eddies or pools (Howard & Cuffey, 2003) and areas with stones or boulders that likely shelter mussel beds from scour during flood events (Vannote & Minshall, 1982); indeed, mussel beds appear to occupy the most stable areas of the river bed (May & 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 & 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 & Milleman, 1977; Karna & 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 & Brim Box, 1999; O’Brien & 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 & Milleman, 1978). Potential native and nonnative 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 & Milleman, 1977; Karna & Milleman, 1978; Steg, unpublished observation cited in Jepsen, et al., 2012).

Systems:Freshwater
Continuing decline in area, extent and/or quality of habitat:Unknown
Generation Length (years):9-45

Use and Trade [top]

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 [top]

Major Threat(s):

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. 

Conservation Actions [top]

Conservation Actions:

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 nonviable should be prioritised, as should protection from and remediation of known threats.


Citation: Blevins, E., Jepsen, S., Brim Box, J. & Nez, D. 2016. Margaritifera falcata. The IUCN Red List of Threatened Species 2016: e.T91109639A91109660. . Downloaded on 28 February 2017.
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