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Montana Field Guides

Western Pearlshell - Margaritifera falcata

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Species of Concern

Global Rank: G4G5
State Rank: S2
* (see State Rank Reason below)

Agency Status
USFWS:
USFS: SENSITIVE
BLM:
FWP Conservation Tier: 1


 

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State Rank Reason (see State Rank above)
This species is widespread in geographic area, but is declining in terms of area occupied and the number of sites with viable individuals; populations showing repeated reproduction (at least several age classes) are now the exception rather than the rule. Montana currently has only 14 "excellent" viable populations out of ~200 known locations (Stagliano 2010).
  • Details on Status Ranking and Review
    Western Pearlshell (Margaritifera falcata) Conservation Status Review
    Review Date = 09/18/2008
    View State Conservation Rank Criteria
    Population Size

    ScoreU - Unknown

    CommentUnknown.

    Range Extent

    ScoreE - 5,000-20,000 km squared (about 2,000-8,000 square miles)

    Comment5000-20,000km (2,000-8,000square miles)

    Area of Occupancy

    Score - Rank factor not assessed

    Comment200-1000 km (125-620 miles) linear river

    Length of Occupancy

    ScoreLD - 200-1,000 km (about 125-620 miles)

    Long-term Trend

    ScoreC - Substantial Decline (decline of 50-75%)

    CommentSiltation and stream temperature increases with loss of riparian shading and lower snowpack probably contributed to some decline

    Short-term Trend

    ScoreC - Rapidly Declining. Decline of 30-50% in population, range, area occupied, and/or number or condition of occurrences

    Comment

    Threats

    ScoreB - Moderate and imminent threat. Threat is moderate to severe and imminent for a significant proportion (20-60%) of the population or area.

    CommentClimate Change, increasing stream temperatures and lower snowpack could seriously impact the habitat that this speces exists in

    SeverityModerate - Major reduction of species population or long-term degradation or reduction of habitat in Montana, requiring 50-100 years for recovery.

    Comment

    ScopeModerate - 20-60% of total population or area affected

    Comment

    ImmediacyHigh - Threat is operational (happening now) or imminent (within a year).

    CommentOngoing steadily

    Intrinsic Vulnerability

    ScoreA - Highly Vulnerable. Species is slow to mature, reproduces infrequently, and/or has low fecundity such that populations are very slow (> 20 years or 5 generations) to recover from decreases in abundance; or species has low dispersal capability such that extirpated populations are unlikely to become reestablished through natural recolonization (unaided by humans).

    Comment

    Environmental Specificity

    ScoreB - Narrow. Specialist. Specific habitat(s) or other abiotic and/or biotic factors (see above) are used or required by the Element, but these key requirements are common and within the generalized range of the species within the area of interest.

    CommentLong-lived and low fecundity--Host fish relationships are species specific, thus when the host fish (native Westslope Cutthroat Trout) is in decline, this mussel species suffers-

 
General Description
The western pearlshell is Montana's only coldwater trout stream mussel, and the only native mussel found on the west-side of the state (west of the continental divide). Populations of this mussel on the east-side of the divide in Montana reflect the distribution pattern of it's host fish (westslope cutthroat). The shell of M. falcata is elongate, compressed, dark colored, and slightly concave on the ventral edge, oftentimes erosion marks are prominent on the umbo region (see photo). It has weakly developed teeth and a purple nacre (see photo). The normal size is 50 to 85 mm with larger older specimens surpassing 10 cm. In Montana, this species is similar to Ligumia recta, which is larger, thicker, has better developed teeth, pink nacre and occurs only in warmer rivers much further downstream. Since these species occur in very different habitats the likelihood of misidentifying M. falcata in Montana is virtually impossible.

Diagnostic Characteristics
"Similar to M. margaritifera except smaller (maximum length 125 mm) with purple rather than white or whitish nacre, relatively smaller anterior pseudocardinal tooth in left valve, and muscle scars in beak cavity entirely visible from below. Hermaphroditic, whereas in M. margaritifera the sexes are separate" (Clarke 1981: 250). Shell morphology: The shell of M. falcata is elongate, compressed, dark colored, and slightly concave on the ventral edge, oftentimes erosion marks are prominent on the umbo region (see photo). It is the smallest mussel species in Montana and the only one known west of the continental divide. It has weakly developed teeth and a purple nacre (see photo). The normal size is 50 to 85 mm with larger older specimens surpassing 10 cm. In Montana, this species is similar to Ligumia recta, which is larger, thicker, has better developed teeth, pink nacre and occurs only in warmer rivers much further downstream. Since these species occur in very different habitats the likelihood of misidentifying M. falcata in Montana is virtually impossible.

General Distribution
Montana Range


 


Distribution Comments
Margaritifera falcata is found in Alaska (SNR), California (S2S3), Idaho (S3), Montana (S2), Nevada (SNR), Oregon (S3), Washington (S4), Wyoming (SNR), British Columbia (S5) and have been extirpated from Utah (SH) (Nature Serve 2006). Their range is widespread in area, but spotty in viable population coverage. Western MT was originally known as part of species" range; status of this area for the species currently needs further work (Frest and Johannes 1995). In Idaho, the historical range includes sites in the Snake, Coeur d’Alene, Lost, and Salmon River drainages (Frest and Johannes 1997, Frest 1999). Viable populations of pearlshells are persisting in north Idaho in the Coeur d’Alene, St. Joe, and St. Maries Rivers (Stagliano, 2011 unpublished USFS data). In central Idaho, populations with good viability occur in the Clearwater, Selway, Lochsa, Pashimeroi, Lost, Salmon and Little Salmon rivers and in Hells Canyon. In south Idaho, populations are thought to be extant in the upper tributaries of the Snake River, including the Blackfoot River (Frest and Johannes 1997, Frest 1999). Montana’s populations of M. falcata, in contrast to Idaho’s, may be significantly contracting and becoming less viable with stream decreased flows, warming and degradation. Previously reported mussel beds in the larger rivers (Blackfoot, Big Hole, Bitterroot, Clark Fork, etc.) are extirpated from those drainages or are at such low densities, long-term viability is unlikely. This mussel species appears to have crossed the continental divide in Montana from west to east with its salmonid host, the westslope cutthroat trout, Oncorhynchus clarki lewisi (Gustafson 2001). This is the only native trout in the Missouri River headwaters and this explains how M. falcata has reached the Smith River drainage downstream of the Missouri River. Reports of the eastern M. margaritifera in Montana are apparently due to the mistaken assumption that a mussel could not cross the continental divide.

Summary of Observations Submitted for Montana
Number of Observations: 271

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density

Recency

 

(Records associated with a range of dates are excluded from time charts)



Migration
Sedentary as adults, rarely move more than a few meters. As larvae (glochidia on the fish gills), they use their fish host for disperal upstream or downstream to other suitable habitats.

Habitat
The species is found in cool and cold running streams that generally have a low to moderate gradient and are wider than 2 m; perferrable habitat is stable sand or gravel substrates. It is found in hard as well as soft water, unlike M. margaritifera (Clarke 1981). In large Idaho river systems (Salmon and Clearwater River Canyons), M. falcata, attains maximum density and age in river reaches where large boulders structurally stabilize cobbles and interstitial gravels. Boulders tend to prevent significant bed scour during major floods, and these boulder-sheltered mussel beds, although rare, may be critical for population recruitment elsewhere within the river, especially after periodic flood scour of less protected mussel habitat. In Idaho's Salmon and Snake River canyon, where reaches are aggrading with sand and gravel, M. falcata is being replaced by Gonidea angulata.

Food Habits
Freshwater mussels are mostly filter-feeders, siphoning in floating particulate organic materials (small plant or animal) from the water column and straining out the particles and expel the strained water. Pedal feeding with the foot muscle has also been observed, mostly in juveniles and younger age-classes.

Reproductive Characteristics
"The gravid period is from mid May to late June. Hermaphroditic. The glochidia have not been described. Host fishes are the chinook salmon, rainbow trout, brown trout, brook trout, speckled dace, Lahontan redside, and Tahoe sucker. The greatest longevity so far determined for M. falcata is about 67 years, but older specimens probably occur" (Clarke 1981:250). Nearly all mussels (Unionidae) require a host or hosts during the parasitic larval portion of their life cycle. Hosts are usually fish species and hosts for M. falcata in this region were typically and historically Oncorhynchus spp. (Chinook Salmon, Westslope Cutthroat trout, Steelhead, etc.), but Salmo and Salvelinus (introduced spp.) and even Rhinicthys and Catostomus (dace and suckers) are reported to be suitable.

Management
Margaritifera falcata has recently become a Sensitive Species for the USFS (2010), and has been ranked at risk (S2) in Montana since 2008. Despite earlier sharing the same state rank with Idaho, Montana’s populations have shown more dramatic declines (Gustafson, pers. comm., Stagliano 2010) and were downgraded to S2 from S2S4 after more intensive sampling in 2007 and 2008 documented few viable populations in the state (Stagliano, 2010). This species is widespread in geographic area, but is declining in terms of area occupied and the number of sites with viable individuals; populations showing repeated reproduction (at least several age classes) are now the exception rather than the rule (Frest and Johannes 1995, Stagliano 2010). Clarke (1993) asserted that "over-utilization of water resources by man" is responsible for the extirpation of this species in Utah. Individuals of this species can be quite long-lived; populations could exist undetected at low levels for many years without any reproduction. Global Short Term Trend: Declining (decline of 10-30%); Global Short Term Trend Comments: Taxon is declining, in terms of area occupied and number of sites and individuals (Frest and Johannes 1995; Hoving 2004). This species has likely recently been extirpated from the Umatilla River in Oregon (Brim Box et al. 2003). Global Long Term Trend: Substantial to moderate decline (decline of 25-75%); Global Long Term Trend Comments: All 11 localities (perhaps 9 populations) in Utah have been extirpated; it formerly occurred in the northern third of the state (Oliver and Bosworth, 1999).

Threats or Limiting Factors
Eutropification due to agricultural runoff and siltation from improper agricultural practices are typical problems for many of the rivers in this species' range; impoundments and diversions are also continued threats (Frest and Johannes 1995). Specific threats to all populations of M. falcata, include extensive damning and diversion of rivers for irrigation, hydroelectric, and water supply projects has much reduced the WA, OR, ID, and CA range of this species. Heavy nutrient enhancement, siltation, unstable substrate, or similar problems extirpate populations. Much of the middle Snake River in Idaho is rapidly becoming eutropified, due to agricultural runoff, fish farms, and urbanization along the river corridor. Much of the river is impounded behind a series of small dams; this is also detrimental for cold-water species such as this taxon. Fine sediment influx, generally from the same causes, is also a major problem. A recent (1994) landslide impacted some of the historic sites in the Snake River basin. In the lower Columbia River region, threats include impoundments; continued siltation and other impacts on the few remaining sites with habitat characteristics approximating pre-impoundment conditions on the lower Columbia.

References
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Citation for data on this website:
Western Pearlshell — Margaritifera falcata.  Montana Field Guide.  Montana Natural Heritage Program.  Retrieved on September 18, 2014, from http://FieldGuide.mt.gov/speciesDetail.aspx?elcode=IMBIV27020
 
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