NPS / Pete Wintersteen Freshwater musselsThere are 41 native species of freshwater mussels that live in the St. Croix National Scenic Riverway, one of the largest and most diverse groups of these creatures in the United States. All of these mussel species have been present since before European settlement, making them a rare example of an intact community of river animals. Though they may not look like much more than rocks, mussels are central to keeping river water clean and healthy. Mussels are also important because they are uniquely dependent on a network of organisms and specific water conditions in the river. Freshwater mussels are some of the most threatened animals, with over 70% of species declining as they face threats from invasive species, habitat loss, dams, and pollution. 
NPS / Pete Wintersteen What are mussels?At a glance, mussels may not seem exciting, but these animals are more active than you may guess at first. Mussels belong to a group of animals called mollusks, which also includes oysters, slugs, and octopuses. Mussels are invertebrates, meaning that they don’t have an internal skeleton. Instead, they produce a hard, external shell to support and protect their soft bodies. Scientists classify mussels as “bivalves” because their shells have two parts, which hinge open and shut. Inside their shells, mussels have gills that work as powerful filters to pull nutrients, algae, and bacteria from the river’s flowing water. One mussel can filter up to 15 gallons of water each day! Because they are filter feeders, mussels play a crucial role maintaining high water quality for other animals in the river. This way of eating also means that mussels are especially sensitive to pollutants, which can quickly build up in their bodies. Mussels also have feet that anchor them in place in the flowing river water. Their feet help mussels move around, although they move very slowly and often remain in the same location for years. 
NPS / Pete Wintersteen Mussel Life CycleOne reason that mussels are threatened animals is because of the unique conditions they need to reproduce and survive. Female mussels must find a host species, typically a fish or amphibian, to carry their larvae for several weeks. The hosts provide essential nutrients for the larvae during this time. The hosts can also swim these offspring miles upstream, giving native mussel populations a wider range of places to grow and an advantage over invasive species that can’t move upriver the same way. Most mussel larvae, called glochidia, spend 1–3 months riding on the gills or fins of a host animal. Nutrients from the host’s blood help these mussels develop their internal organs. After this time, the young can survive on their own and drop to the river bottom. If they land in a suitable area, with sand or gravel and flowing water, they will grow into adults. Some types of mussels can live for 80 years or more. Mussels’ unique reliance on host species makes mussels vulnerable to change. For example, winged mapleleaf mussels will only survive and reproduce if channel catfish carry their offspring — no other fish will do! If environmental challenges threaten a particular host species, mussels will be threatened, too. To attract the right host, mussels use a variety of flashy behaviors called displays. They wave body parts with streamers or fringe that look like minnows to catch the attention of host. When the host swims close enough, the mussels then eject their young, which then latch onto the host. 
NPS / Pete Wintersteen Where Can You Find Mussels?The St. Croix National Scenic Riverway has a variety of underwater habitats that are ideal for freshwater mussels. You can find mussels in boulder fields, gravel beds, sandbars, or mucky backwaters. But remember, to protect federally endangered and state threatened species, it is illegal to take any live mussel or empty mussel shell from the St. Croix and Namekagon rivers. It is also prohibited to move mussels from one place to another because they can suffocate if they are replaced upside down! 
NPS / Pete Wintersteen What is the National Park Service doing to protect native mussels?The requirement of specific host fish, competition from exotic species, and requirements for flowing, unpolluted rivers means that mussels species face many threats. One of the major threats to native mussels are invasive zebra mussels. These mussels attach themselves to anything that’s hard, like rocks, bridges, boats, and even native mussels. They have reproduced quickly since they first appeared in 1990’s and make difficult for native species to survive because they all compete for food. The National Park Service works with a variety of partners to monitor zebra mussels and prevent their spread. One way to protect the river is by doing boat inspections, which help ensure that those who want to enjoy the river aren’t bringing any damaging hitchhikers with them. Collaborative projects across multiple agencies are also trying to strengthen native mussel populations. For example, in 2022, a batch of young winged mapleleaf mussels raised in ponds and tanks by scientists became the first since 2005 to survive a winter when released back into the river. |
Last updated: May 20, 2026