Freshwater mussels are the incredible unsung heroes of our lakes, rivers, and streams. Mussels may be cryptic and easily overlooked, but they play key roles in the ecology and food webs of aquatic ecosystems.
The soft body of a freshwater mussel is enveloped and protected by a pair of strong, hinged, calcium-rich shells (also called valves), giving this group their common family name of “bivalves”. Mussels hold themselves in the gravel, sand, or sediment at the bottom of a lake or stream bed using a strong muscular foot, which can be pushed out through the opened shell and anchored in the substrate. They can use this foot to slowly crawl along by extending the foot in front of their body, embedding it in the substrate, then pulling the rest of their body and shell along. Mussels also move vertically within the substrate. Typically they are only partially buried in the substrate to feed, but can burrow down deeper to avoid being washed out during floods or to stay warmer in winter, and sit up higher in the substrate during breeding season. Most mussels will move little more than a few hundred feet as adults, which makes them extremely vulnerable to changing habitat conditions.
Freshwater mussels are nature’s great living water purifiers. They feed by using an inhalent aperture (sometimes called a siphon) to filter small organic particles, such as bacteria, algae, and detritus, out of the water column and into their gill chambers. An exhalent aperture is used to expel filtered water, fecal material, and undigestible particles back into the habitat. Juvenile (young adult) mussels can also engage in “pedal feeding”, using cilia on the foot to sweep food particles from the substrate into their shell. Mussels’ filter feeding activities improve water quality and chemistry, benefiting the rivers and lakes in which they live for a variety of other aquatic and terrestrial life. Their fecal pellets provide food for aquatic insects and other invertebrates that are at the heart of the aquatic food web, and mussels themselves are eaten by everything from sturgeon and waterbirds to raccoons and otters. Mussels are often found in dense aggregations, called beds, making them a living part of the substrate, providing shelter and habitat for aquatic snails, caddisflies, midges, and a variety of other aquatic invertebrates that fish rely on for food. Water chemistry and clarity is often greater in the vicinity of a mussel bed, and mussel beds can be “hotspots” for nutrient cycling and aquatic invertebrate abundance and diversity.
It seems only fair that mussels help improve habitat and food quality for fish, because without fish, there would be no freshwater mussels. Mussel reproduction relies primarily on fish to act as hosts for larval mussels (also called glochidia). When a gravid female mussel releases her glochidia, they must find a suitable host fish to attach to or they will die. Attachment to a host fish endures not only larval mussel survival, but also provides a means for these slow-moving creatures to hitch a ride hundreds of miles into new waters, where the glochidia can drop off and settle down into the substrate to begin their long slow life of filter-feeding. In most cases the glochidia cause no harm to their hosts.
Humans have a long relationship with freshwater mussels as well, as they were a major food resource for some prehistoric peoples along the Mississippi and Columbia River drainages. Mussel harvest by tribes in the Pacific Northwest dates back more than 10,000 years; mussel shells were also used as tools and ornamentation. Unfortunately, many of the more recent interactions between humans and mussels have had severe negative impacts on native mussel populations and habitats. Impaired water quality, habitat, destruction, loss of native host fish species, and damming of rivers have made freshwater mussels the most at-risk group of animals in North America.
The Pacific Northwest Native Freshwater Mussel Workgroup is an association of practitioners working in Northwestern states and Canadian provinces to foster awareness and appreciation of the roles and importance of freshwater mussels, and to increase mussel research and conservation efforts. Founded in 2003, the workgroup maintains a list of local and regional mussel resources, and provides technical assistance and recommendations to the scientific community and general public. Please join in on the fun!