EVANSTON – Resting on the bottom of the Bear River in southwestern Wyoming is a palm-sized mussel pushing 100 years old.
You read that right. There’s probably more than one mussel living in a Wyoming waterway born between the first and second world wars.
They’re called western pearlshell mussels, and they’re the longest-lived invertebrate in Wyoming.
If you’re surprised, it’s OK. Most Wyoming residents, even avid anglers who spend long days on the water, may not realize the state boasts not one but seven native mussel species.
Mostly we just read about zebra and quagga mussels, the invasive species from eastern Europe feared and reviled in the U.S. due to their tendency to clog waterways, ruin water treatment plants and strip nutrients from entire ecosystems.
But our native mussels aren’t those mussels. As critical as it is for zebra and quagga mussels to stay out of Wyoming waters, it’s equally important that native mussels stay in them.
“They basically vacuum the water in streams. They suck in water through one syphon, clean it up, and that’s what they eat, and put clean water back,” said Lusha Tronstad, an invertebrate zoologist with the Wyoming Natural Diversity Database, a service and research unit of the University of Wyoming. “If mussels are abundant, they can clean more water than what comes down the stream.”
Mussels are more diverse in North America than anywhere else in the world. The continent boasts 300 species, yet 70% of them are at risk of extinction either locally or nationally.
Most Wyoming mussels are likely faring better than their brethren across the country, but at least one species may have disappeared.
That’s why in July, the Wyoming Game and Fish Department, along with Tronstad and other researchers, waded out to deep holes in the Laramie River near Grayrocks Reservoir to deposit a couple thousand plain pocketbook mussels. The 7-inch long mollusks are native to the drainages but haven’t been seen alive since 2008.
“The ecosystem services mussels provide are amazing,” said Stephen Siddons, a Game and Fish fisheries biologist. “It’s also part of our responsibilities as managers of these aquatic systems to maintain biodiversity, and this is a species that needs help right now.”
Strange names, even stranger habits
For water creatures rarely seen and seldom considered, Wyoming’s freshwater mussels have some colorful names and even more interesting life cycles.
Take the fatmucket mussel. It’s a cousin to the plain pocketbook, living in northern Wyoming and reaching about 7 years old. It can grow to 5 inches long and uses a lure to attract fish in order to reproduce.
The giant floater, meanwhile, is up to 10 inches long (hence the first part of its name) and has a tendency to float when it perishes. Tronstad calls the species cryptic because they live up to 30 years and look like rocks covered in algae.
The white heelsplitter has a wing that sticks up (use your imagination on its name origin), and is thriving in northeast Wyoming where it carpets the bottom of the Belle Fourche River, Tronstad said.
The cylindrical papershell lives in the same waters as the plain pocketbook. The California floater in the Bear River, with white and pink insides, is the only other mussel not faring as well as biologists would like.
Wyoming even has one family of native fingernail clams, but that’s for another story.
Wyoming mussels require a host fish to propagate. Reproduction goes something like this: A male mussel releases thousands of sperm into the water; the female sucks up the sperm as she does everything else, using it to fertilize her eggs until she’s ready to release larvae, called glochidia, into the wild. But glochidia can’t live long in the water without a hard shell, bobbing around susceptible to any number of threats. So they look for a fish host.
Some larvae float until they find a fish’s gills to latch onto, absorbing just a bit of nutrients from the fish while growing until they drop to the river bottom and the fish swims along, no worse for the wear.
But other mussels, like the plain pocketbook and the fatmucket, have a more clever adaptation. Instead of sending their glochidia into the water column to seek a host, they open their shell a little bit and display a fleshy piece that looks, we promise, like a minnow. The plain pocketbook even has two markings on either side resembling eyes, and a fleshy faux-tail. When a hungry fish swims by and bites, the mussel sends thousands of glochidia into the fish’s mouth to latch onto its gills.
Only certain fish species will work – the western pearlshell and California floater require trout – which is one reason biologists stress the importance of maintaining native fish.
Slow movers with only one foot
Overharvest was once an issue for mussels in the Midwest — their shells were fashioned into the nation’s button supply — but hasn’t likely been a problem in Wyoming. Here, the limiting factors are likely abundance of host fish, damming and, in particular, water fluctuations.
If water drops dramatically as it does in the lower North Platte River in eastern Wyoming, mussels struggle to breathe.
“Dams are changing the hydrology of rivers. Mussels can’t deal with the rapid water changes,” Tronstad said. “They have a muscular foot they stick out of the shell and pull themselves along.”
As she likes to tell students during class visits, mussel transportation is, “one foot moving slowly.”
That was likely part of the problem for the plain pocketbook mussel. It’s native to the North Platte and Laramie Rivers, but when Tronstad and Siddons ran aquatic DNA tests in the rivers they found no trace.
So Siddon received 2,500 plain pocketbook mussels from the North Platte fish hatchery in Nebraska and reintroduced them in July. Another 3,000 are slated for this summer. He hopes they will be more successful than their predecessors in the drainage. The river has plenty of host fish including sauger, walleye, largemouth and smallmouth bass, sunfish and bluegill. The water quality is also suitable.
Those concerned about mussels can’t do much to help them, Tronstad said, other than being careful not to introduce a nonnative species like zebra or quagga mussels — which reproduce at exponential rates, don’t require host fish to grow and outcompete native mussels. We should also be careful of any discharges into watersheds.
So the next time you catch a glimpse of a minnow-like creature on the bottom of a creek, look again, it could be a mussel ready to give birth.
Buy a Wyoming mussel guide – The University of Wyoming’s Biodiversity Institute created a guide to Wyoming’s mussels available for $4.50 at wyobiodiversity.net.
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