Montana Field Guide

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 declining distribution pattern of it's host fish (Westslope Cutthroat Trout). 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 typical size is 50 to 85 mm (2-3 inches) with larger older specimens surpassing 10 cm. In Montana, this species is similar to the introduced mussel, 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.

"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). 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.

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

The species is found in cool and cold running streams that generally have a low to moderate gradient and are wider than 2 m; preferred 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.

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.

The gravid period is from mid-May to late-June. Glochidia packets were observed from a pearlshell in the Boulder River, MT (Jefferson Basin) on June 23rd, 2015 when the water temperature was 13.5 degrees C (Stagliano 2015, unpublished data). Can be Hermaphroditic. Host fishes range-wide are the Chinook Salmon, Rainbow Trout, Brown Trout, Brook Trout, Speckled Dace, Lahontan Redside, and Tahoe Sucker. In Montana, the native host fish was solely the Westslope Cutthroat Trout. The greatest longevity so far determined for M. falcata is about 67 years, but older specimens probably occur" (Clarke 1981). 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.

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, personal communication, 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).

Collection for human consumption can be a threat to some populations but harvest or possession of any freshwater mussel in Montana's Western and Central Fishing Districts is prohibited (see Montana Fishing Regulations).

Eutrophification 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 damming 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 suffering from eutrophication 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.