St. Croix Creature Feature: River is refuge for endangered mussel

Spectaclecase species is subject of intensive research and protection.




7 minute read

An array of spectaclecase mussels awaiting measurement before being returned to the water. (Greg Seitz, St. Croix 360)

Last week, biologists with the Wisconsin Department of Natural Resources were on the St. Croix near Somerset Landing, continuing an extensive project to document where spectaclecase mussels live.

The type of freshwater clam was added to the federal list of Endangered Species in 2012. It was listed as threatened by the state of Minnesota in 1996, and Wisconsin has also designated it as endangered.

Like many mussel species, the spectaclecase carries a unique name that hails from the 19th century, when it was first named by European scientists. Its size and elongated, curved shape does resemble something you would use to safely carry around your precious eyeglasses.

While the species was designated as federally-endangered seven years ago, it wasn’t until this June that its full protection was promised. Responding to a lawsuit, the U.S. Fish and Wildlife Service agreed to designate “critical habitat” for spectaclecase and three other mussel species. Identifying where they live and why they live there is a key step to preventing extinction.

“Freshwater mussels are America’s most endangered group of animals, so it’s fantastic that these four incredibly important creatures will get habitat protection,” said Tierra Curry, a senior scientist at the Center for Biological Diversity. “Those safeguards will benefit these mussels along with the health of rivers across 18 states.”

Once habitat is designated, any federally-funded or permitted project in those areas will have to ensure the area is not damaged. The St. Croix’s Wild and Scenic River rules already provide it important protections here.

The USFWS now must propose protected habitat within five years.

Biologists Jesse Weinzinger (nearest diver), Jake Winkler (farthest diver), and Lisie Ketchel work on a spectaclescase mussel survey on the St. Croix River, August 14, 2019 (Greg Seitz, St. Croix 360)

There is still a lot that is not known about the creatures. Only two years ago, Minnesota DNR scientists finally figured out what fish species they use during reproduction (goldeye and mooneye). 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.

One thing scientists do know is that spectaclecase like to live among rocks and rubble on the bottom of a river. That means they are probably less encountered on the St. Croix River than other species which are commonly found in sandy areas that attract people.

It also means they are attracted to human structures, like the historic wing dams that were once built on the St. Croix (as well as the Mississippi and other rivers) to force water toward the center of the river, improving boat navigation.

Working a rocky slope on the Wisconsin shoreline just above Somerset Landing, DNR divers found numerous specimens. They spend 15 minutes diving, grabbing all mussels they see, then surface and bring them to the boat. All species are counted before being returned to the river, and the spectaclecase get measured and aged, all of it written down in careful notes.

St. Croix 360 will feature more about multiple mussel research projects underway on the river next month. For now, here’s more about a fascinating but reclusive animal that makes it home in the Wild and Scenic St. Croix.

Spectaclecase (Cumberlandia monodonta)

Map of current range of spectaclecase mussels, significantly reduced from its historic home.

From the U.S. Fish and Wildlife Service:

The spectaclecase is a freshwater mussel that the U.S. Fish and Wildlife Service listed as an endangered species. Endangered species are animals and plants that are in danger of becoming extinct. Threatened species are animals and plants that are likely to become endangered in the foreseeable future. Identifying, protecting and restoring endangered and threatened species are primary objectives of the U.S. Fish and Wildlife Service’s endangered species program.

What is a spectaclecase mussel?

The spectaclecase is a large mussel that can grow up to 9 inches in length. The shape of the shell is elongated, sometimes curved, and somewhat inflated, hence its name.

Historically, the spectaclecase was found in at least 44 streams of the Mississippi, Ohio and Missouri River basins in 14 states. It has been extirpated from 3 states and today is found in only 20 streams. The spectaclecase’s current range includes Alabama, Arkansas, Illinois, Iowa, Kentucky, Minnesota, Missouri, Tennessee, Virginia, West Virginia, and Wisconsin. With few exceptions, spectaclecase populations are fragmented and restricted to short stream reaches.

The life cycle of the spectaclecase is complex and includes a stage parasitic on fish or other host species. Males release sperm into the river current. As females siphon water for food and respiration, they also siphon sperm that fertilizes their eggs. Within special gill chambers, fertilized eggs develop into microscopic larvae called glochidia. After they mature, female mussels expel the glochidia, which must then attach to the gills or fins of a specific species, usually a fish, to continue developing into a juvenile mussel.

If glochidia successfully attach to a host, they mature into juvenile mussels, and then drop off. If they land in a suitable area, glochidia grow into adult mussels. Using fish (or other aquatic species) as a host allows the spectaclecase to move upstream and populate habitats it could not otherwise reach. The host species for spectaclecase are not known.

As a group, mussels are long-lived, with individuals surviving up to several decades, and sometimes up to 100 to 200 years. The oldest documented spectaclecase was thought to be 70 years old.

Spectaclecase mussels are found in large rivers where they live in areas sheltered from the main force of the river current. This species often clusters in firm mud and in sheltered areas, such as beneath rock slabs, between boulders and even under tree roots.

Feeding Habits:
Adult spectaclecase are suspension-feeders, siphoning water and feeding on suspended algae, bacteria, detritus, microscopic animals and dissolved organic material. Adult mussels spend their entire lives partially or completely buried within river bottom substrates.

What are threats to the spectaclecase mussel?

Population losses due to dams have contributed more to the decline and potential extinction of the spectaclecase than any other factor. Dams affect both upstream and downstream populations by disrupting seasonal flow patterns, scouring river bottoms, changing water temperatures and eliminating river habitat. Large rivers throughout nearly all of the spectaclecase mussel’s range have been dammed, leaving short, isolated patches of habitat between dams. Spectaclecase mussels likely depend on a fish species, or other aquatic species, to move upstream.

Because dams block fish passage, mussels are also prevented from moving upstream. This isolates upstream populations from those downstream, leading to small, unstable populations, which are more likely to die out.

Small Population Size and Fragmentation:
Most remaining populations of spectaclecase are small and geographically isolated. Small populations remaining in short sections of rivers are susceptible to extirpation from single catastrophic events, such as a toxic spill. Also, this level of isolation makes natural repopulation of areas that once supported mussels impossible without human intervention.

Poor land use practices, dredging, intensive timber harvests, highway construction, and other activities accelerate erosion and increase sedimentation. Sediment that blankets a river bottom can suffocate mussels since they cannot move to avoid the impact. Also, large amounts of sediment in the water column reduce the ability of mussels to remove food and oxygen, which can lead to reduced growth, reproduction and survival.

Adult mussels are easily harmed by toxins and degraded water quality from pollution because they are sedentary (they tend to stay in one place). Pollution may come from specific, identifiable locations such as accidental spills, factory discharges, sewage treatment plants and landfills, or from diffuse sources like runoff from fields, feedlots, mines, construction sites and roads.

Contaminants may directly kill mussels, but they may also indirectly harm spectaclecase by reducing water quality, affecting the ability of surviving mussels to reproduce and lowering the numbers of host fish.

Dredging and channelization have profoundly altered riverine habitats nationwide. Channelization physically changes streams by accelerating erosion, reducing depths, decreasing habitat diversity, destabilizing stream bottoms and removing riparian vegetation.

Nonnative Species:
The invasion of the nonnative zebra mussel into the United States poses a serious threat to native mussels. Zebra mussels proliferate in such high numbers that they use up food resources. They attach to native mussel shells in such large numbers that the native mussel cannot open its shell to eat or breath.

What is being done to conserve the spectaclecase?

The spectaclecase was added to the list of threatened and endangered species, giving the species full protection under the Endangered Species Act. The ESA provides protection against practices that kill or harm the species and requires planning for recovery and conservation actions.

Prevent or Slow Spread of Zebra Mussels:
States and tribes are working to prevent the spread of zebra mussels to areas such as the northern portions of the St. Croix River by enforcing aquatic nuisance species laws, monitoring, and providing information for boaters at water access sites.

Monitoring and Research:
Many of the states that have spectaclecase populations and some federal agencies are conducting surveys and funding research to find out specifics about this mussel’s life history requirements and threats to its survival.

What can I do to help prevent the extinction of animals and plants?
Learn more about how the destruction of habitat leads to loss of endangered and threatened species and our nation’s plant and animal diversity. Discuss with others what you have learned.

Help improve water quality in your local streams by minimizing use of lawn-care chemicals and properly disposing of or recycling hazardous materials found in your home, like batteries, paint, car oil, and pesticides.

When boating, please follow rules established to prevent the spread of exotic pests like the zebra mussel.

Join a conservation group or volunteer at a local nature center, zoo, or wildlife refuge.


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St. Croix Creature Feature: River is refuge for endangered mussel