Special Series: Flexing Mussels

The St. Croix River is one of the best mussel (aka freshwater clams) laboratories in America, because there is an incredible number of species present, and because their populations are typically large.

This summer, I joined three separate teams of scientists doing mussel research on the river. The adventure began with wading and swimming in the St. Croix searching for Wabash pigtoe, just below the river’s rocky gorge…

The day was cloudy and muggy, so our crew of citizen and professional scientists couldn’t wait to get in the water. 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 experienced college students, a National Park Service biologist, and in the middle, Mark Hove, making it all happen.

It was a warm July afternoon, and a few folks were pushing canoes and kayaks into the water and heading downstream when we finally waded into the welcoming waters.

The group was assembled to search for a specific 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, 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, and worked with Minnesota Master Naturalists to complete the study.

The amateur malacologists that 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. There were multiple Master Naturalists, two teenage girls from Minneapolis’s Southwest High School’s robotics team, and their two adult advisors, a pair of Hove’s former students who still help out when they can, and a writer.

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 mussel’s watery habitat while they hunted 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 could often tell what kind of mussel he grabbed before he even brought it to the surface, while the rest of the group asked him about each one they found. He 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.

Soon, the mussel scientist was diving underwater to work in deeper areas, and would come up wiping water from his face and studying his finds, while usually fielding questions from the crowd.

At first I tried to wade around while carrying my camera bag and notebook, only observing the action, but I kept wanting to creep deeper into the water. That way, I could hide more skin from the mosquitoes, and I wanted to see if I could find a mussel. I stashed my gear on shore and joined the group feeling around the rocky river bottom.

The water felt great. It was cool enough to refresh, but as warm as I’d ever want. It pushed insistently against me, and over the riverbed, where mussels sucked in the water, filtered out sediment and nutrients, and spat it out again.

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. The sheer number of the creatures, and the number of species residing in a patch of river the size of a basketball court, was astounding.

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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. Everything except Wabash pigtoes were immediately returned to the river, while the Wabash were hauled to St. Paul to reveal their secrets.

The Master Naturalists in the crew included Joel Donna, an assistant professor of education at the University of Wisconsin-River Falls. He was working with Hove as a true partner on the project, offering valuable support to the study, and gaining the unique experience of working on rigorous research and publishing peer-reviewed articles in scientific journals.

While scientists know the names of all the mussel species in the St. Croix River, and know what each one looks like, there remains much that is a mystery about individual species. Hove and his collaborators represent the ongoing process of study and discovery.

Mussels have unique lives, and their complicated methods of reproduction were once again the main question with this project. Their survival for eons has depended on relationships with specific fish species that share their waters, and make it possible for mussels to reproduce.

Female mussels eject their eggs in a variety of ways, each method meant to get the larvae onto the gills of fish — often a specific species. The young grow on the 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.

After collecting numerous specimens, the group helped Hove with one more task, dragging a large seine 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.

The mussels would also be put into tanks with fish, to see if they could naturally “infest” the fish with their young.

The Iowa DNR hoped the research would reveal a link between a fish species and the Wabash pigtoe that could explain the mussel’s decline. Or, it might rule out issues with its fish hosts, pointing to other causes. Very little is still known about many species of mussels, including questions about habitat, water quality, and relationships to fish.

Later, I learned juvenile mussels are particularly sensitive to ammonia and nitrate in the water — byproducts from factory farms that have quadrupled in Iowa during the past two decades (and are now seeking to expand into the St. Croix River region). There’s no evidence yet that CAFOs are to blame for the Wabash pigtoe’s woes, but studies like this could help understand how runoff harms river ecosystems.

Once we had the fish, mostly minnows, Hove carefully put them into coolers complete with aerators in the trunk of his car, to keep them alive on the trip back to campus. He patiently situated them as he continued to feed the mosquitoes.

Months later, Hove and Donna stood in front of more than 100 people at the St. Croix River Research Rendezvous to share their project and findings. The presentation was not just about the insights gained into the Wabash pigtoe, but into the potential of partnerships between scientists and volunteers.

The research team identified 14 new minnow host species for the Wabash Pigtoe, and two new killifish species hosts. The knowledge which will help Minnesota, Iowa, and places across North America protect the mussels and their river habitat, enjoyed by many people and creatures. It could also inform a hatchery program where mussels could be raised before being released into the wild.

But a project like this one is not just about exchanging volunteer labor for scientific experience, or only getting useful data. Hove said while volunteers not only supply much-needed hands in the river looking for mussels, and more help around the lab setting up and monitoring the experiments, they also provide fresh points-of-view.

“Working with citizen scientists provides invaluable new perspectives,” Hove told the audience. “It helps me think about my research in new ways.”