Science Corner
Cheryl Lowe, NOSC Board Member

When someone mentions freshwater mussels, I immediately think of invasive zebra and quagga mussels and the problems they cause. I had no idea there were also native freshwater mussels until last month when my husband and I sought shelter from the pouring rain in a small natural history exhibit at Silver Falls State Park in Oregon. The exhibit label next to a small mussel shell mentioned its dependence on salmon, and my interest was piqued. Do they live on the Olympic peninsula? How exactly are they connected to salmon? Where might I find one?

One of the best on-line resources I found is “Freshwater Mussels of the Pacific Northwest”, by Ethan Nedeau, Allan K Smith and Jen Stone. They introduce these fascinating creatures by noting “To the casual observer, a freshwater mussel may look no different than a stone. Mussels do not move very far during their adult lives; they may inch their way along the bottom or slowly bury themselves if the need arises…[But] their young attach to unwitting fish that carry them to new places in a watershed—over waterfalls, across lakes, up and down rivers from headwaters to tidewaters…”

The western pearlshell (Margaritifera falcata) is one of several species of freshwater mussels found on the Olympic Peninsula. It prefers cold, clean creeks and rivers where salmonids spawn. Unlike the invasive freshwater species mentioned above, the western pearlshell has a dark brown to black outer shell and can grow up to 5” long. (Zebra and quagga mussels are light tan or striped and less than an inch long). Native mussels prefer sand, gravel and cobble substrates, especially in stable areas of a streambed such as the downstream side of large boulders or near banks where the current is gentle and substrates are more stable.

WA Dept of Fish and Wildlife (WDFW) reports that this native species is found in Pacific drainages from California to BC and southern Alaska, including parts of the northern Rocky Mts. “The western pearlshell is long-lived [up to 100 years!], sedentary, and sensitive to environmental change, so it is considered an excellent indicator of water quality. …Native freshwater mussels have been greatly affected by dams and annual water drawdowns, as well as degraded water quality resulting from development and agriculture. Many historical sites no longer support mussels, and many local populations no longer successfully reproduce.”

Life Cycle

According to the Nedeau et al, “[The western pearlshell] is the most common freshwater mussel species in the Pacific Northwest...Recent conservation concerns about western pearlshells closely mirror … the loss and decline of Pacific salmon fisheries. Both need clean cold streams and rivers, and western pearlshells need salmon and trout to reproduce.”

Here’s how it works. In the spring, male pearlshells release sperm into the water, which is then taken in by nearby females to fertilize their eggs. Within a few weeks, eggs grow into larvae, known as glochidia. They are released into the flowing water and must attach themselves in short order to the gills or fins of chinook, coho or sockeye salmon; native cutthroat or rainbow trout; or a few other freshwater fish species. The glochidia form a cyst around themselves and are carried upstream by the host fish, maturing as they travel for several days to a month, depending on water temperature and other factors. When the juvenile mussels have grown enough, they release themselves from their host fish, burrow into the sediment and start growing. The mussels may take nine to twelve years to reach sexual maturity (Nedeau et al).

Conservation Concerns

A few on-line resources provide geographic location information on western pearlshells, including iNaturalist. If you are interested in looking for freshwater mussels, you can use the resources below to find identification guides, best techniques for observing and recording information without doing damage, and how to report what you find.

The WDFW website says that the western pearlshell mussel is uncommon and is declining or extinct over much of its range due to habitat alteration and pollution. They welcome the assistance of interested citizens by noting, “If you see this species, please share your observation using the WDFW wildlife reporting form” [see below].

I may not ever find a western pearlshell (although I’ll certainly be looking), but I now see an even wider and more complex web of species benefiting from the habitat restoration work that NOSC has undertaken and continues to tackle on the many streams and rivers of the Olympic Peninsula. Thank you, NOSC.

Resources

“Freshwater Mussels of the Pacific Northwest” by Ethan Nedeau, Allan K Smith and Jen Stone. https://molluskconservation.org/Library/Maps/pdfs/Pacific_Northwest_Mussel_Guide.pdf

Xerces Society Resources on Mollusk Conservation: https://www.xerces.org/endangered-species/freshwater-mussels

WDFW: https://wdfw.wa.gov/species-habitats/species/margaritifera-falcata#resources

WDFW Reporting Observations: https://wdfw.wa.gov/get-involved/report-observations

Western Fisheries Resource Center: https://www.usgs.gov/centers/western-fisheries-research-center/news/secret-lives-freshwater-mussels-learning-about

Image 1: https://www.usgs.gov/media/images/underwater-image-margaritifera-falcata-western-pearlshell-mussels-a-river-oregon

Image 2: https://www.inaturalist.org/guide_taxa/724641

Image 3: https://xerces.org/sites/default/files/publications/10-030.pdf

Image 4: https://lsintspl3.wgbh.org/en-us/lesson/ilwhyy18-il-ilmussels/4

Image 5: https://www.inaturalist.org/guide_taxa/724641