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. Last week, the adventure began with wading and swimming in the St. Croix searching for Wabash pigtoe. We now head downstream and look at another species, another set of scientists…

It’s quiet while the divers are below. Lisie sits in the boat and sorts and counts and measures and records 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.

Up here, there’s just occasional bird song on a lazy August morning. The river is quiet, the sun is still behind the steep bluff we are anchored below. Two or three boats go past us over a couple hours.

The crew of Wisconsin Department of Natural Resources biologists, Lisie Kitchel, Jesse Weinzinger, and Jake Winkler, spent several days this summer doing this in the St. Croix, as well as around Wisconsin. Their work requires entering the underwater world, studying and surveying secretive creatures, and providing the hard data that drives management and protection.

‘Wimpy’ species

Today, I’ve paddled my kayak over to meet up with them along this rocky shoreline on the Wisconsin side of the river, just upstream from Somerset Landing and Marine on St. Croix. It

They were looking for an 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 its holdouts.

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Anyone who has swam at a beach on the river has probably come across a mussel in the sand. It wasn’t a spectaclecase.

This species prefers rocky cavities, tucking itself 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 Lisie says, “wimpy.”

Science for stewardship

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 they are often found near wing dams, bridge pilings, and other human structures.

That’s one reason the divers were looking for spectaclecase. Because the mussels are a federally-endangered species, the government must protect them when doing anything where they live.

Spectaclecase are also found on the Mississippi River, and the U.S. Army Corps of Engineers needs to take their presence into account when repairing or maintaining the wing dams that help ensure a deep enough channel for barge and other commercial navigation — and attract spectaclecase. The Minnesota DNR was focusing on the Mississippi this summer, while Wisconsin took the St. Croix.

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 they found the creatures. Their efforts will also improve population estimates, and hopefully learn more about the species.

When it’s been 15 minutes, Lisie 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.

15 minutes, 16 species

Lisie 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. Standing waist-deep, they deliver the bounty to Lisie.

She tells them there were 16 species of mussels in their last batch. In an area the size of a pickleball court, 16 different kinds of mussels. 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.

Jesse says they have been starting at the bottom of the rocky slope below us, which continues from the bluff high above to four or five feet underwater. They move larger rocks around to expose the crevices where spectaclecase hide. They grab everything they find.

Questions and clues

Besides the fact 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 was discovered in 2017. As I wrote in a Creature Feature back in August:

Questions remain about their life cycle, what their exact habitat requirements are, and more. Learning more about them is another important part of protecting the species.

While we now know which fish species the mussel depends on for reproduction, plenty of questions remain. 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 (featured in last week’s story) 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 specifically 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.

“Trash fish” falsehood

“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. “As we’ve learned, ecosystems are complex with numerous important inter-relationships between species and their environments. Goldeye and Mooneye, like sturgeon, are the last members of a unique, relatively unchanged group of fishes. As these species decline, the well-being of Spectaclecase follows.”

With mussels filtering the river water, providing food for several other species, and otherwise playing their part in the ecosystem, their fate will be forever entwined with the health of their host fish populations, specific habitat, and clean water.

While the divers are below, Lisie 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.

Like trees, they stop growing each winter, and have the rings to prove it. Some of these are 20 to 30 years old.

Once the crew had worked over the site, the divers get back in the boat and they head downstream to an old wing dam on Crunchberry Island, in front of Marine on St. Croix. More than 100 years ago, a crew working for 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 may now be 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. A first report from the study will be released in March.

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