Western pearlshell mussels in the Tahoe Basin

Western pearlshell mussel exterior shell.     Western pearlshell mussel interior shell.

Figure 1. Photograph of Margaritifera falcata shell exterior (top) and interior (bottom) © Ethan Jay Nedeau, reproduced from Freshwater Mussels of the Pacific Northwest (Nedeau et al. 2005).

The western pearlshell (Margaritifera falcata) is a freshwater mussel that is native to the Tahoe Basin. The species ranges from Alaska south to central California and east to Nevada, Wyoming, Utah and Montana (Hovingh 2004, Jepsen et al. 2010). Western pearlshell mussels inhabit cold creeks and rivers with clean water, where you can find them wedged between cobbles, partially burrowed in sand, underneath mats of aquatic vegetation, or beneath undercut banks. They have an average lifespan of 60 to 70 years, some living more than one hundred years, making them one of the longest-lived animal species on Earth (Nedeau et al 2005).

Many historic populations have been drastically reduced in size from dense beds to a few isolated individuals (Vannote and Minshall 1982, Hovingh 2004, Strayer et al. 2004, Howard 2008, 2010). There is still, however, a lack of historical abundance data for this species, making it difficult to document the level of decline across its range (Jepsen et al. 2010). Threats to populations of western pearlshell mussels include anthropogenic impacts on watersheds such as chemical pollution, impoundments, channel modification, siltation, introduction of exotic species, and the decline of native host fish (Jepsen et al. 2010, Nedeau et al. 2005).

Taylor (1981) hypothesized that western pearlshell had been eradicated from much of its native range in California, and Hovingh (2004) concluded that the species had been extirpated from Nevada. Western pearlshell were once numerous in the Truckee River with 20,000 individuals identified in 1942 (Murphy) but only 120 found in 2007 (Howard 2008). Today, the only known populations of the western pearlshell mussel in the Tahoe basin are in the Upper Truckee River, Trout Creek, and the Truckee River. PLEASE DO NOT handle or disturb individuals due to their sensitivity and rarity in the Lake Tahoe basin.

A Western pearlshell mussel bed in the Upper Truckee River.

 

Figure 2. A western pearlshell mussel bed in the Upper Truckee River.

Western pearlshell mussels can grow up to five inches long, and are light brown (juveniles) to dark brown or black. The interior of the shell is usually purple, salmon-colored, or pink (sometimes white) (Nedeau et al. 2005). They are easily distinguished from non-native bivalves due to their larger size and difference in shape. Asian clam (Corbicula fluminea) is a non-native freshwater bivalve that is established in Lake Tahoe. They are small, averaging less than 1.5 inches, with shells ranging from white to light or dark brown (FWS 2014). Zebra and quagga mussels, not currently present in Lake Tahoe, are also considerably smaller in size than the western pearlshell.

A comparison of adult and juvenile western pearlshell with Asian clam.

 

Figure 3. A comparison of adult and juvenile western pearlshell with Asian clam.

Size of zebra and quagga mussels compared to a dime.

 

Figure 4. Size of zebra and quagga mussels. Photo courtesy of California Department of Fish and Wildlife.

A quick and easy identification key to distinguish native and non-native mollusks in the Lake Tahoe Basin can be found at: https://tahoercd.org/wp-content/uploads/2013/03/Native-vs.-Invasive-Mollusk-Identification-Key.pdf.

If you find western pearlshell mussels, please do not handle or disturb individuals due to their sensitivity and rarity in the Lake Tahoe Basin. Moving wildlife is a violation of California and Nevada state laws. Unusual occurrences such as piles of dead mussels or shells, groups of mussels in a dry section of river or creek channel, or sightings outside of the species’ known range should be reported to LTBMU aquatic biologists by calling the Forest Supervisors Office at 530-543-2600.

References

FWS; US Fish and Wildlife Service Nevada Fish and Wildlife Office. 2014. Asian Clams. Accessed September 2, 2018. https://www.fws.gov/nevada/nv_species/invasive_species/asian_clams.htm.

Hovingh, P. 2004. Intermountain freshwater mollusks, USA (Margaritifera, Anodonta, Gonidea, Valvata, Ferrissia): geography, conservation, and fish management implications. Monographs of the Western North American Naturalist 2:109-135.

Howard, J. K. 2008. Strategic inventory of freshwater mussels in the northern Sierra Nevada Province. Final Report by Western Mollusk Sciences, San Francisco, CA to US Forest Service PSW Regional Office, Vallejo, CA. 65 pp.

Howard, J. K. 2010. Sensitive freshwater mussel surveys in the Pacific Southwest Region: Assessment of Conservation Status. Final Report by Western Mollusk Sciences, San Francisco, CA to US Forest Service PSW Regional Office, Vallejo, CA. 60 pp.

Jepsen, S., C. LaBar, and J. Zarnoch. 2010. Margaritifera falcata species profile. The Xerces Society for Invertebrate Conservation. 24 pp.

Murphy, G. 1942. Relationship of the fresh-water mussel to trout in the Truckee River. California Fish and Game 28(2): 89-102.

Nedeau, E., A. K. Smith, and J. Stone. 2005. Freshwater mussels of the Pacific Northwest.  United States Fish and Wildlife Service. 45 pp.

Strayer, D., J. Downing, W. Haag, T. King, J. Layzer, T. Newton, and S. Nichols. 2004. Changing perspectives on pearly mussels, North America’s most imperiled animals. BioScience 54(5):429-439.

Vannote, R. L., and G. W. Minshall. 1982. Fluvial processes and local lithology controlling abundance, structure, and composition of mussel beds. Proceedings of the National Academy of Sciences 79:4103-4107.





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