Turn back the calendar 200 years or more to a summer day in the place we now call Minnesota. Then step into a river in your bare feet.
You might want to proceed gingerly as you wade. In many places you stand a good chance of stepping on the sharp edges of native mussels living on the river bottom. In a few places where they form colonies, or beds, you won't be able to avoid mussels, which were superabundant in segments of waterways all around the state, according to DNR mussel biologist Bernard Sietman.
"There are historical accounts of people walking a mile in a river and never setting their foot down without stepping on a mussel," he says.
The wide, slack stretch of the Mississippi known as Lake Pepin was home to a large and dense mussel population, says fellow DNR mussel biologist Mike Davis, citing records and accounts from the button industry, which once harvested and made clothing buttons from Lake Pepin mussel shells.
Today it's a different story. Davis and Sietman have surveyed for native mussels all over the state in more modern times and rarely encountered such populations.
"Most of the rivers don't have anything near that high a density," says Davis.
Human activities, from the building of dams to the pollution of waterways, have depleted many native mussel populations in the southern half of the state and extirpated at least two species, extensive surveys have shown. Twenty-eight of Minnesota's 51 native mussel species are listed as endangered, threatened, or of special concern. We now live in a Minnesota where native mussels are far less often encountered, let alone appreciated, and where an infamous invasive species, the zebra mussel, is often the first thing that comes to mind when people hear the word mussel.
A small team of DNR biologists is working to change this state of affairs. Doing painstaking, patient work to raise and propagate native mussels for restoration, they are beginning to undo what has been done to these animals. As this crew of scientists returns more and more mussels to waters where they once lived, they expect that the environmental benefits those mussels bring will follow.
Behold the Mussel.
Mussels live out of sight at the bottom of warm-water lakes and rivers. They pretty much mind their own business, which may explain why people don't often think much about them. But look more closely and you'll find that mussels are fascinating both in their own right and for all the things they do for the waters where they live.
Mussels are animals, but they might not seem like they are because they don't have eyes, ears, arms, legs, or many of those things that we identify as animal-like. When we think of mussels or their kin, clams and oysters, we tend to focus on the shell (or the pearls that some of them make), but not—unless you're a shellfish connoisseur—the fleshy and surprisingly sophisticated organism within.
Like clams and oysters, mussels are mollusks, the second- largest group of animals in the world. As members of this clan, mussels are closely related to snails and octopuses. They are known as bivalves for the two halves, or valves, of their shell. Armless and legless, they nonetheless have a single "foot" that they can use to move around and anchor themselves to the bottom of water bodies. Some of them produce tiny pearls every bit as beautiful as larger pearls from their kin the oyster. Some species can live to be more than 100 years old.
"They're the Methuselahs of the river, right up there with the ancient sturgeon and snapping turtles," Davis says.
Mussels basically serve as small, efficient, organic pumps and filters: Their main pursuit in life is pumping water through their bodies and over gills that filter oxygen and particles from the water. Tiny plankton, fungus, bacteria, and other organic matter are captured and consumed as food.
Filtration is a happy byproduct of mussels' feeding activity, cleaning and clearing the water. (Zebra mussels, unchecked by natural controls, can multiply this filtering effect to the point where it can harm the lake and aquatic life.) Dense aggregations of native mussels can positively alter the water chemistry in their microenvironment, changing the nitrogen-phosphorus ratio to favor more species of algae. And the mollusks deposit unused food particles as pseudofeces, or false poop, that's a food source for other macroinvertebrates.
Freshwater mussel beds have been equated to the coral of the ocean for their tendency to be a hub of activity and sustenance.
"Other invertebrate life is richer in and amongst mussel beds, and more abundant, than in a stream without mussels," says Davis. "And that means there's more food for fish. They really create habitat and food resources for fish. In larger numbers, we think they can stabilize the riverbed."
A Wild Life.
Mussels have a complex life history. To reproduce, female mussels lure their host fish into close proximity, usually by vividly mimicking prey, then fool the fish into harboring their young by releasing parasitic larvae called glochidia into the fish's gills. (One enterprising species actually snaps shut on the fish's head to forcibly accomplish this feat.) Attached to the gills, the glochidia metamorphose before dropping off as juvenile mussels—all without harming the host.
"Somehow," says Davis, "an animal that's completely blind produces a lure for a fish that looks just like a food item to the fish."
For some fussy mussels, only one species of fish is a suitable host. Others use many fish as hosts. And some species have their own preferences for habitat. The mussels known as lake floaters, for example, are exclusive to lakes, while rock pocketbooks are found in large rivers. The tiny fingernail clam is a generalist that lives in many types of waters, including wetlands and seasonal ponds.
The challenge, then, for scientists who want to restore mussels is to propagate them by mimicking this entire convoluted life cycle and then putting juvenile mussels into habitat with the right conditions—and the right host fish—for them to thrive to adulthood and naturally reproduce. Inside an unassuming pole building in Lake City, not far from the great former mussel beds of Lake Pepin, DNR mussel propagation biologists are, remarkably, performing just this trick.
At the Center for Aquatic Mollusk Programs, or CAMP, water burbles through pumps and tubes and filters and into racks and rows of aquariums, large tanks, and fish hatchery–style raceways. Some tanks hold gravid mussels. Some hold baby mussels. Some hold host fish. Staff often wear foam clogs or rubber boots on a well-drained concrete floor made for splashes and spills. Along one wall, computers, microscopes, and science lab equipment are set up for measuring, monitoring, and data crunching. In another room, gleaming new fieldwork boats, racks of scuba gear, and various aquatic gear stand ready to roll out of a large garage bay. The feel is part science lab, part craft brewery, part James Bond villain hideaway.
Working in this lab and in the field—which in their case means the water—they are attempting to restore mussels to the watersheds of four state rivers: the Mississippi, the St. Croix, the Cannon, and the Cedar. A tight-knit unit by virtue of their small size and shared mission in a niche field, they sometimes call themselves the Clam Fam.
The Mussel That Shall Not Be Named
Let's play a word association game. The word is mussel. What word does it make you think of?
Is it zebra?
If so, you're not alone. The highly invasive species known as the zebra mussel appears, unfortunately, to be the best-known mussel in Minnesota. The invaders have tainted the native mussels' reputation by association, which doesn't do any favors for the professionals trying to restore the good kind.
"Zebra mussels are bad in their own way," says DNR mussel biologist Bernard Sietman, referring to the ecological harm they cause, "but they're also bad because people conflate zebra mussels with native mussels. A lot." At the Minnesota State Fair, where he meets a cross section of Minnesotans at the DNR building, "I still have to spend probably half or more of my time distinguishing between the two."
Sietman created a simple poster to help out. Basically, it conveys that "this is a zebra mussel, and this is a native mussel, and this is why they're different, and these are bad and these are good."
To break it down a bit more, the invasive species can grow unchecked and harm the environment and property. Zebra mussels cluster profusely on hard underwater surfaces from docks to boats to power plant intakes, causing costly damage. They can attach to and kill native mussels and consume food particles that would nourish mussels or other aquatic life. In contrast, native mussels live in harmony with the waters where they live. They help, not harm, their local aquatic ecosystem and grow no more abundant than their environment can support.
Don't Fear the Creeper
The English-language common names of native Minnesota mussels are whimsical and creative. Here are 10 of our favorites.
Seated before a microscope trained on a tray of four baby mussels, Pletta views them on a tablet device that serves as a monitor. The mussels are surprisingly active, opening and closing vigorously enough to make them tumble and move. These are endangered spectaclecase mussels, a species that the team is preparing to restore to some state waters.
Sietman and Davis, whose work on mussel surveys and restoration predates the CAMP lab by more than a decade, made a big leap in spectaclecase restoration when they discovered in 2017 that the host fishes for this mussel are the mooneye and the goldeye, two closely related members of the mooneye group of fish. Other scientists had been searching for years for this answer, which won Sietman, Davis, and University of Minnesota biologist Mark Hove a federal science award.
The new facility enabled the discovery, says Sietman, with its ability to house larger holding tanks. But it was scientific sleuthing that put together the pieces of the puzzle that led to the mooneye and the goldeye.
Sietman knew, from sampling that he and Davis had done, that spectaclecase mussels, native to the St. Croix River, were not reproducing upstream of the St. Croix Falls dam, but they were reproducing downstream of the dam. Many fish that live in the St. Croix had already been ruled out as hosts, so Sietman contacted retired DNR fish biologist Konrad Schmidt.
"He has an excellent handle on the distribution and status of fishes in the state, and he gave me a list of the species above and below St. Croix Falls dam," says Sietman. "These fishes popped out like a sore thumb," occurring downstream of the dam but not up. "So we promptly went out the next season and got some mooneyes from the Mississippi River and tested them." They got positive results. After more testing and confirmation, they verified the results and solved a pressing mussel mystery.
From Lab to Field.
On a tour of the lab, Pletta walks a visitor through the propagation process, a precise and elaborate series of steps that must be done just so to achieve results. A row of tanks holds gravid female mussels. A wall-size rack of aquariums hosts one fish per tank, where they will be the lucky recipient of a glochidia inoculation. Larger tanks contain darting schools of fish that will be used as hosts or in lab trials to identify hosts.
Many of the propagation systems were designed by Pletta, based on systems she worked with at a Virginia fish hatchery, and assembled by her and former technician Tricia Wagner.
"I was kind of forced to learn to become a plumber," says Pletta. She got assistance and expertise in building her first system from the Minnesota Zoo, a partner with CAMP in mussel restoration. She freely calls both the systems she designed and the mussels she propagates her "babies" and dotes on them accordingly.
High tech in its totality, the lab is low tech in many of its components, which were cobbled together from various sources on a limited budget. Some tubs and tanks come from farm equipment stores. The UV and biofilter, made for a large pond, came from a donor via the Minnesota Zoo. Fish hatchery equipment including raceways and air agitators came from the closed DNR French River fish hatchery near Duluth.
Everything from temperature to water chemistry is closely monitored. As in a fish hatchery, disease and contamination are constant concerns. Someone is always on duty watching the CAMP lab, even when others are out in the field.
And field work is frequent. In addition to their own propagation pursuits, the CAMP team does mussel-related contract work to supplement their patchwork of funding. They do mussel surveys at river bridge construction sites for the state Department of Transportation and provide expertise and services for entities from county highway departments to the U.S. Geological Survey.
In mussel restoration, scientists are playing the long game. It takes years from starting the propagation to observing results in rivers where mussels have been placed. But signs of success are showing for at least two endangered mussels, the Higgins eye pearlymussel and the winged mapleleaf.
"We think we've successfully reestablished a Higgins eye population in the Mississippi up here," says Davis, referring to the section of the river between St. Paul and Lake Pepin, where these mussels were released starting in the early 2000s. "We've found a few juveniles and some that are young enough to have resulted from our efforts. And we've established a population of the winged mapleleaf in the St. Croix River at Hudson. They appear to be producing viable larvae, and we have found some juveniles there.
"We think that if we can get native mussels back into our rivers, it will make our rivers more resilient to things like climate change, for example. We can clean up the water and make our rivers better and healthier."
Out on Their Own.
The CAMP team finishes off its raising of juvenile mussels by putting them into natural waters inside hole-filled plastic tubs, where they acclimate to conditions and grow to a size suitable for life in the wild. On a fall day, CAMP crew members are at East Side Lake in Austin, checking in on two mussel species that they're readying for release in spring in the nearby and interconnected Cedar River. Pletta and Wagner are in chest waders, meandering in the muck, when Pletta halts and says, "Here they are."
She and Wagner, their jacket sleeves rolled up, plunge their neoprene-gloved hands into the chilly October water and pull up a tub. Each holding a handle, the two walk the tub to shore, remove zip ties holding it shut, record a code from a tag, and begin dumping dark sediment from the tub through a mesh sieve box and into a different tub.
"We don't know how these are going to look," says Pletta.
These mussels are a species called muckets. If all goes well, they will be released in the spring in the Cedar River, where they used to live. But it is not yet certain that all is well.
"This is our first attempt to rear them in this lake," says Pletta, peering down into the roiling water and silt rushing through the sieve.
Soon, she exclaims: "Oh! There's mussels! And fish. I see mussels. There's mussels in there!"
"They look good, too," adds Wagner.
Mike Davis holds up a mucket that grew from 7 millimeters a year ago to 23 millimeters today.
"This is a genuine Cedar River mucket," says Davis, beaming. "They've been missing from this river for more than 100 years."
CAMP technician Morgan Freeburg has set up a table on shore for data entry and mussel tagging. As the wetsuited duo continues to pull up young muckets, they come over to towel dry, measure, and mark each one with a tiny yellow tag that will allow them to track it when it is released into the wild.
Sietman and Secrist pull up in a DNR truck and quickly don wetsuits. Together they enter the water and pull up more totes. These contain the other mussel they're restoring in the Cedar, the black sandshell. A larger species, it still surprises the biologists with its hearty growth and the sheer number of mussels in the tubs.
"They're so big," says Wagner, looking at the handsome black sandshells in one load.
"These are even bigger!" she says at the next.
Pletta emits a squeal of delight. For a mussel propagation biologist, nothing is quite so satisfying as seeing mussels growing and thriving and preparing to go out into the world on their own. "This is like my baby project," she says. "So to have mussels to release in the spring—it's great."