Montana Mussels: Shrouded In Mystery
By Diane Tipton, Montana Fish, Wildlife & Parks Statewide Information Officer
Western Pearlshell Mussels
Western Pearlshells from East Fork Fisher River
Montana’s freshwater mussels can be an important source of food for raccoons, muskrat, otter and other animals who live near the trout streams and prairie rivers the mussels inhabit. But perhaps most importantly they are an important indicator of water quality and healthy fish populations.
The state is home to three native mussel species—the western pearlshell, Margaritifera falcata; the fatmucket or Lampsilis siliquoidaea; and the giant floater, or Pyganodon grandis. Mussels are mollusks within the class of bivalvia, meaning literally two valves or shells joined by a hinged ligament.
"Freshwater mussels filter water to sift out organic material and micronutrients. Their internal system is a mirror image of what is in the surrounding water including pollutants," said David Stagliano, the aquatic ecologist with the Montana Natural Heritage Program, the state’s source on the status and distribution of plants and animals, especially those of conservation concern.
Stagliano is leading a three year study of mussels initiated in 2007 when Montana’s Comprehensive Fish & Wildlife Conservation Strategy, an analysis of the state’s 600 species and their habitats conducted by Montana Fish, Wildlife & Parks, found the state didn’t know enough about the status of freshwater mussels to monitor changes in their numbers and the quality of their habitats.
In 2007, Stagliano and crews sampled 266 stream reaches in 18 river basins; particularly focusing on the western pearlshell, Montana’s only native trout-stream mussel. He also revisited 40 sites where the western pearlshell mussel had previously been reported based on data as much as 70 years old. More than 60 percent of the sites did not have mussel populations present.
Most species of mussels live between 10 and 40 years, though some in Siberia are said to live over 100 years; our western pearlshell has been shown to live on average 60-80 years. Freshwater mussels are aged by growth rings on their shells.
To move, which isn’t a common urge, mussels open their shell enough to put out their single foot and crawl a few inches at a time. Reproduction is even easier. The male releases sperm to float downstream. The female filters the sperm out of the water as she filters food and the sperm fertilize the eggs which grow into microscopic glochidia, or parasitic larva.
Each mussel species has a different adaptation for performing the next amazing step—the mussel lures a particular species of fish with a body part that simulates a favorite food of that species of fish. When the fish tries to feed on the "lure," the female mussel expells nearly microscopic glochidia that then attach to the fish’s gills, fins and skin. The glochidia feed off the fish for up to three months then drop off to live on their own, leaving the fish unharmed.
This host-fish relationship is a truly ingenious way for sedentary animals to disperse to suitable upstream areas where the fish may carry them.
MNHP’s future plans include research on how mussels are responding to warming water temperatures and the potential reintroduction of the western pearlshell mussel into westslope cutthroat restoration sites. The pearlshell’s native fish host was the westslope cutthroat trout.
To learn more about Montana’s freshwater mussels, visit Montana’s online field guide produced by FWP and MNHP at http://fieldguide.mt.gov .
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MUSSELS ANYONE? Early people may have used native freshwater mussels from Montana’s rivers as a food source some local archeologists say.
Stephen Aaberg of Billings has published an article in Archaeology in Montana describing a site between Ulm and Great Falls on the Missouri River with what he reports is the closest thing to a musselshell midden in Montana. Middens are piles of remains or waste left behind by humans or other animals. Freshwater musselshell middens indicate humans may have ’processed’ mussels for food or other uses.
"Interestingly, the site near Great Falls also contained lots of bison bone and a very large feature that may have been associated with mussel processing," Aaberg said. Aaberg said a few stone tools were also found at this site, which is on private property today.
Archaeological digs in the Three-Forks area of the Missouri River studied by other archaeologists in 2000 revealed two species of mussel in large numbers, the fatmucket and western pearlshell.
Native people used freshwater mussel shells for beads, pendants and other decorations, and it is believed they may have consumed them for food too.
Sites near the Marias River’s confluence with the Missouri were dominated by a single mussel species, the fatmucket, which can tolerate warmer water temperatures. Shell fragments have been collected within known bison kill or jump sites indicating human camps in these areas for extended periods of time.
While early people may have eaten freshwater mussels, it is unlikely mussels will ever regain their appeal as a food source for two main reasons. Today’s untreated water may contain pollutants that mussels, during the feeding process, filter through them; and secondly ’rubbery’ is a flattering description of a freshwater mussel’s flesh.
For more on archaeology in Montana, contact the Montana Historical Preservation Office at 406-444-7719, or by email to: swilmoth@mt.gov .
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