Freshwater Mussels

More than 350 species of freshwater mussels once lived in North America. Of those, 25 species of freshwater mussels have historically been found throughout the Chesapeake Bay region. Unlike oysters, which live in saltwater, these mussels live on the bottom of rivers and streams where the water is fresh. Every Chesapeake Bay state and the District of Columbia has freshwater mussels. The Susquehanna, Potomac, Rappahannock, York, and James Rivers are all home to at least 17 or more species. Many of these species have very colorful names, like the creek hillsplitter, paper pondshell, and Eastern pearlshell.

Freshwater mussels cannot move very far on their own, but they’ve come up with a very cool way to get around: They hitch a ride on fish! In order to reproduce, adult freshwater mussels create a specialized ‘lure’ that looks like a small fish. They wave the lure around in the water to attract a bigger fish. When the fish bites the lure, it pops like a water balloon and releases tiny baby mussels that cling to the fish’s gills. In this way, the babies travel upstream and settle in new places. Once there, some species can live as long as 100 years.

Freshwater mussels, like oysters, filter the water as they search for food. This filtering can improve water quality and even help remove nitrogen—a major pollutant in the Chesapeake Bay—before it moves downstream. Beds of freshwater mussels on the bottom of streams also improve habitat by providing homes for many types of aquatic ‘bugs,’ called macroinvertebrates, that fish and other species eat.

In the past 200 years, the number of freshwater mussels and the number of places where they are found have both declined dramatically, according to Chesapeake Bay scientists. The federal government lists three of the region’s species as either endangered or threatened, while individual Bay states list 10 species. Endangered species include the dwarf wedgemussel, triangle floater, brook floater, Atlantic pigtoe, yellow lampmussel, Eastern lampmussel, green floater, tidewater mucket, Eastern pondmussel, and James spinymussel. However, there is still a lot we don’t know about the current state of freshwater mussels because monitoring is limited.

Most freshwater mussels are used to living in free-flowing rivers and streams. Dams can create deep, lake-like reservoirs where many mussels can’t survive. They also change how water flows, leading to shifts in water temperature, water depth, and the amount of food that flows downstream. By preventing fish from moving upstream, dams can also cut off the connection between mussels and the fish they need to reproduce. Development and forest loss can also change how water flows through rivers and streams and threaten the places where mussels live.

Much of the pollution that threatens the health of the Chesapeake Bay threatens freshwater mussels. For example, mussels are very sensitive to ammonia, a form of nitrogen. Low oxygen driven by excessive nitrogen and phosphorus, as well as sediment, can also harm mussels. Other pollution of concern includes heavy metals, chlorides, pharmaceuticals, and microplastics.

Climate change is creating increasingly extreme storms and droughts. This can create rapid, intense increases in water flowing into rivers and streams or alternatively create long periods of low water, both of which can threaten mussels.

Invasive species like zebra and quagga mussels can directly harm or out-compete native freshwater mussels. Scientists have also documented unexplained, large-scale die-offs of freshwater mussels that they believe could be due to a combination of factors including disease.