Adapted from a three-part series that originally appeared on St. Croix 360, we take a look back at the 2019 mussel research season on the St. Croix River.
“The uncommon richness of mussel species in the St. Croix parallels the uncommon richness of the flora and fauna of the watershed as a whole.”– U.S. Fish & Wildlife Service
Many mussel species almost should not exist. Their lives are so precariously dependent on such specific conditions that their continued ability not only to survive but to reproduce, generation after generation, is remarkable.
Even minor disruptions to their homes can cause the whole thing to fall apart. Pollution can poison them. Subtle changes in water chemistry can weaken the creatures, especially by killing young mussels. Increased sediment can smother them. Climate change can disrupt critical ecosystem connections. These are challenges that confront most animals, but mussels face another risk: they might lose access to a fish species they need to carry their babies for their first stage of life.
The threats are many, but somehow these species have not just survived over the ages, but become successful specialists. It’s one reason why the St. Croix National Scenic Riverway, with its 41 species, is special. Not only is there remarkable species diversity, but the individual populations are typically large.
This richness and abundance is why the St. Croix is one of the best mussel laboratories in America. At any given time, there are frequently several studies and projects focused on mussels occurring in the Riverway, with support from National Park Service biologists. In the summer of 2019, I spent time on the river with three research teams.
July: Citizens scientists study special St. Croix creatures
The day was cloudy and muggy, the banks were buzzing with mosquitoes, and only the river offered respite, as well as the unique chance to engage in some St. Croix River mussel research.
Swatting bugs in the parking lot by the boat landing at Wisconsin Interstate Park were high school students, volunteer naturalists, a couple of experienced college students, a National Park Service biologist, and, in the middle, Mark Hove, making it all happen.
The group was assembled to search for a species of mussel in the river for a study Hove is running. We weren’t looking for any of the rare or endangered mussel species that call the St. Croix home, but rather, Wabash pigtoes (Fusconaia flava), which are relatively common.
Their abundance in Minnesota and Wisconsin is notable because one state south, in Iowa, the Wabash pigtoe is disappearing. That state’s Department of Natural Resources wants to know why they thrive up here but are now uncommon in the Hawkeye State, and asked their neighbors to the north for help.
The Minnesota DNR enlisted Hove, who operates a mussel research lab at the University of Minnesota, to investigate. He recruited the ragtag group of assistants to grope along the river bottom, searching for specimens that could lead him to the answers.
The amateur malacologists who gratefully splashed into the waters and submerged themselves neck-deep in the St. Croix to escape blood-sucking insects represented the power of citizen science. It takes many hands to find mussels in murky waters, but the fact is, Hove doesn’t have a team of paid technicians. Anyone who likes swimming and wading in the river can contribute to the effort.
After wading downstream through a stretch of soft sand, we arrived at a lovely bend where the current kept the rocks clean of sediment. Under our feet, we could feel stones ranging in size from racquetballs to volleyballs, while tucked between were the unique organisms we were there to see.
The water was just murky enough from recent rains that the bottom wasn’t visible, so everyone quickly got to work hunting with hands. The most productive areas seemed to be in about three feet of water, so some could keep a foot planted on the bottom as an anchor against the strong current, bend over with head above water, and reach down to feel through the rocks.
Others wore diving masks and snorkels and fully submerged themselves in the mussels’ watery habitat to hunt for pigtoes. National Park Service biologist Allison Holdhusen even wore SCUBA swim fins, but everyone struggled to stay in place against the current while searching.
When someone got hold of a mussel, they brought it to the surface to see what it was.
Hove patiently identified each one, and everyone slowly started to learn to identify a couple species. Hove’s enthusiasm for mussels and rivers is matched by his joy in sharing the magic of mussels and mentoring nascent scientists.
Despite their abundance in the St. Croix, we couldn’t find a Wabash pigtoe for a long time. We pulled up mussel after mussel, and each one seemed to be a new species. We pulled pistol grips, muckets, fatmuckets, heelsplitters, wartybacks, monkeyface, and more. The evocative common names for many mussel species often reflect the era in which they were named by European-American settlers.
We also found a lot of round pigtoe mussels, which are nearly identical to Wabash except the Wabash has a slight indentation along the ridge of its shell. Late in the day, we finally found where Wabash pigtoes were hiding in mucky sand. We collected numerous specimens and considered the trip a success.
Everything except Wabash pigtoes were immediately returned to the river, while the Wabash were put in coolers with aerated water and hauled to St. Paul to reveal their secrets.
While scientists know the names of all the mussel species in the St. Croix River, and know what each one looks like, many mysteries remain.
Mussels have unique lives, and their complicated methods of reproduction were the main question with this project — as with many other studies. Their survival for eons has depended on relationships with particular fish species that share their waters and make it possible for mussels to reproduce.
Female mussels get their larvae onto the gills of fish in a variety of ways. The young grow on this “host” fish for a while before dropping off and completing their development on the bottom of the river.
Figuring out which species of mussels are connected to which species of fish is often critical for conservation.
The last task was meant to provide information on this other side of the relationship. The group helped Hove drag a large net through the water to capture fish. These fish would be taken back to the lab and sorted into aquariums. Then the scientists would watch and see if any of them were carrying baby Wabash pigtoes, indicating the fish was a suitable host species.
In a separate part of the study, some of the collected Wabash mussels would be put into tanks with the fish, to see if they could naturally “infest” the fish with their young.
Ultimately, the research team identified 14 minnow host species for the Wabash Pigtoe, and two killifish species hosts, none of which were previously known to host this mussel species’ juveniles. The knowledge will help Minnesota, Iowa, and places across North America protect the mussels and their river habitat. It could also inform a hatchery program where mussels could be raised before being released into the wild.
August: Searching for spectaclecase
It’s quiet while the divers are below. Lisie Kitchel sits in the boat, counting, measuring and recording the last batch of mussels they brought to her. There are dozens in mesh bags and on a metal tray on the side of the boat.
Bubbles rise to the surface from the two young men submerged in SCUBA gear, and occasionally a flipper or tank pops above the water. They are diving only a few feet deep, staying submerged for 15 minutes at a stretch, bagging as many mussels as they can in that time.
The crew of Wisconsin Department of Natural Resources biologists — Kitchel, Jesse Weinzinger, and Jake Winkler — spent several days this summer doing this in the lower St. Croix and in other selected Wisconsin waters. Their work requires entering the underwater world, studying and surveying secretive creatures, and providing the hard data that informs management and protection.
Today they were looking for a Federally endangered species of mussel called the spectaclecase (Cumberlandia monodonta), as part of a joint project with the Minnesota DNR funded by the U.S. Fish and Wildlife Service.
Spectaclecase mussels are today found in fewer than half the streams where they lived historically, occupying just 20 streams across the eastern United States. The St. Croix is one of their holdouts.
This species prefers rocky cavities, where they can tuck themselves safely out of the strongest current. They are well adapted to this habitat, because their muscle called a “foot,” which emerges from the shell and can pull them along or hold them in place, is relatively weak. Their shells are thin.
Spectaclecase are fragile and delicate. Or as Kitchel says, “wimpy.”
Hiding in nooks and crannies serves them well, providing protection from predators and heavy river flows. But it can also invite certain interactions with people, as the mussels are often found near wing dams, bridge pilings, and other human structures. Because the mussels are a federally-endangered species, the government must protect them when doing anything where they live. Protecting the mussels requires understanding their habitat and needs.
This crew’s work was partly to help inform government actions on the Mississippi River, where spectaclecase are also found. The U.S. Army Corps of Engineers needs to take their presence into account when maintaining wing dams — which help ensure a deep enough channel for commercial navigation, and also happen to attract spectaclecase.
This careful study was in part trying to determine more specifics about where spectaclecase live, which is why the team recorded the depth and flow at each spot where they found the creatures. Their efforts will also improve population estimates, and hopefully tell us more about the species.
When it’s been 15 minutes, Kitchel grabs her hammer.
By controlling the length of time spent on the bottom of the river looking for spectaclecase, the scientists can more accurately estimate total population size and other key information. They stick to 15-minute forays, methodically moving across the river bottom.
Kitchel leans over the edge of the sturdy boat and bangs her mallet on the hull. This is the divers’ signal to ascend. It can be loud underwater with their breathing and the current and other river noises, so sometimes it takes a few times pounding on the boat to get their attention.
The divers come up and take off masks and mouth pieces and rub the water from their faces. They carry bulging mesh bags over to the boat.
Kitchel tells them there were 16 species of mussels in their last batch—in an area the size of an average living room. Many rivers in this region are lucky to have 10 species in their whole length.
The St. Croix River’s tremendous mussel diversity was demonstrated once again, reflecting the health of the river and its watershed.
Aside from the the facts that spectaclecase live in river rubble and are weaklings, they are still mostly a mussel mystery. It was only with complicated laboratory experiments by the Minnesota DNR that the host fish species for spectaclecase — mooneye and goldeye — were discovered in 2017.
While we now know which fish species the mussel depends on for reproduction, plenty of questions remain, particularly about what specific types of habitat they prefer. The Wisconsin DNR dive crew was looking for a few more answers, in the mussel laboratory that is the St. Croix River.
University of Minnesota mussel researcher Mark Hove points out that both of spectaclecase’s host fish species are still considered rough fish, or worse, “trash fish,” by many people and wildlife managers.
Goldeye and mooneye are listed as “rough fish” by Minnesota law, meaning there is almost no management or limits on harvests, including by commercial fishermen. Hove says such an attitude ignores their critical role in the closely-connected ecosystem of the St. Croix River.
“The idea that some species are completely worthless, or trash, is a notion that can be retired with the 1950s,” Hove told St. Croix 360.
While the divers descend again, Kitchel sorts the mussels by species, and then measures their size, determines their age, and otherwise assesses the animal before returning it to the river. Spectaclecase have lines on their shell that give a good idea of age. Some of these are 20 to 30 years old.
Once the crew works over the site, the divers get back in the boat and they head downstream to an old wing dam. These structures were created more than 100 years ago, when the Army Corps of Engineers piled rock at key points along the banks to direct flow toward the channel used by steamboats and other traffic.
The paddlewheelers are now mostly gone from this stretch of river, but the rocks still make a great place for spectaclecase. The divers go down, and bring back more mussels, and more information.
September: A morning with the winged mapleleaf moms
The winged mapleleaf mussel (Quadrula fragosa) has come as close to the edge of extinction as possible without disappearing completely, but it, too, hangs on here. It was once found throughout the Mississippi River and its tributaries, including the nearby Minnesota and upper Mississippi Rivers. All told, the species once inhabited at least 34 river systems in 12 states.
Now, it only lives in four streams in the country: the Ouachita and Saline Rivers in Arkansas, the Bourbeuse River in Missouri, and a small stretch of its former range in the St. Croix, which is the only place where it is known to reproduce.
The growth of young mussels is tied to habitat conditions, particularly water temperatures in late summer. This also affects the movements and behavior of their host fish, the channel catfish, and so the whole cycle does a delicate dance depending on a fragile rhythm.
University of Minnesota mussel researcher Mark Hove told St. Croix 360 that 200 years ago, these mussels were probably found the entire length of the St. Croix. He and others have collected old shells of the species from Wild River State Park to Stillwater. But today, living winged mapleleaf mussels are found from Interstate State Park to William O’Brien State Park, with the majority in just the six miles of river between Interstate and Franconia.
Their future is perilous.
The winged mapleleaf mussel has the highest level of protection possible in the federal Endangered Species Act, and the species is listed as endangered by both Minnesota and Wisconsin. The International Union for Conservation of Nature and Natural Resources classifies it “Critically Endangered.” Their current range in the St. Croix is protected by the National Park Service.
Ensuring they don’t go from endangered to extinct is a priority. A collaborative effort to help the species includes the U.S. Fish and Wildlife Service, the National Park Service, and the Minnesota Department of Natural Resources.
This is why a team of federal biologists donned wetsuits and stepped out of a boat into the St. Croix River one morning in September. Megan Bradley and her colleagues were checking in on a community of winged mapleleaf mothers.
At this site near the Minnesota Interstate Park boat landing, in sight of many thousands of people each year, but hidden by just a few feet of water, these creatures were preparing to do something that is not believed to happen anywhere else on Earth: the propagation of a new generation of winged mapleleaf mussels.
The U.S. Fish and Wildlife Service is trying to help that happen. The agency’s scientists are going so far as to help them reproduce successfully by providing a controlled environment.
The crew clad in wetsuits and snorkels wading in the strong current is part of a careful effort to raise the young in a laboratory. Someday, these young could help establish new self-sustaining populations in other rivers where the winged mapleleaf has been lost, or in other parts of the St. Croix.
It is a tricky process.
First, the mussels in the river must be closely monitored so the females are only transported back to the lab when they are holding onto fertilized eggs. Males eject sperm into the water, where it reaches females, who then embrace their young for a short time. The experts can look inside the shell and tell when they’re ready.
This is when they must be collected and carried back to the lab. Too early and they won’t be fertilized, but too late and the eggs will already be fending for themselves, hopefully having found their way to a channel catfish. If the mussels are ejected before they are taken to the laboratory, they are unavailable for the breeding program.
During this outing, only a few mussels were ready to go back to the lab. The biologists would be back many more times in the weeks ahead to monitor their progress and carry them back to a maternity ward they’ve prepared some 180 miles away.
At the Genoa Fish Hatchery in southwest Wisconsin, the egg-carrying females are placed in a tank with channel catfish, where the larvae hopefully attach.
Once they have entrusted their larvae to the fish, the mother mussels are brought back to the river. The catfish live in an aquarium designed to mimic the natural seasonal cycles of water temperatures. When they drop the mature larvae off their gills, the scientists collect the juveniles and then raise them in individual petri dishes.
It has been slow going. The channel catfish host species was only discovered in 2005. The first mussels were successfully propagated in 2016 and released into the Mississippi River in the Twin Cities. These are important early steps, but progress is slow. Someday the mussels will hopefully thrive across the Mississippi River watershed, just as they do in the St. Croix.