Freshwater mussels are mollusks, just like oysters, clams, snails, slugs, and squid. All freshwater mussels belong to the class  Bivalvia. Mussels are classified by a pair of hinged shells that protect the animal from its environment. The shell is primarily calcium carbonate and protein, and is a non-living portion of the animal, similar to hair or claws on mammals.

Freshwater mussels have ligaments and muscles to hold the shells together and use a meaty  foot to slowly move along and dig into the stream bottom. Freshwater mussels are filter feeders. They have two  apertures, or openings—one that draws in water and food, and one that expels filtered water and waste.

Although they look similar, clams and mussels are different! Freshwater mussels have flat, oblong shells, while clams are more round (closer to symmetrical). Additionally, freshwater mussels are less tolerant than clams to environmental disturbance and have slightly different habitat requirements. North America has a higher diversity of freshwater mussels than anywhere else in the world.

Freshwater Mussels

Why are Freshwater Mussels Important?

  • Freshwater mussels  perform an important ecological service by keeping our streams and rivers clean. They filter out large quantities of harmful algae and bacteria. They also absorb heavy metals and filter silt and fine particulates that harm aquatic ecosystems.

  • They are  indicators of water quality. They cannot escape polluted or disturbed habitat.

  • Mussels are long-lived and can exist in large populations which make them important for the long-term health of streams, rivers, lakes, and ponds. Mussels act like earthworms in a garden, improving overall water quality at the bottom of rivers and lakes.

  • They are a food source for aquatic and terrestrial animals. By-products of filter feeding also provide food resources for plants and aquatic insects.

  • Mussel shells can  provide habitat for fish, crayfish, aquatic insects, and larval salamanders.

  • In addition to their importance in the ecosystem, mussels are an  important part of human culture. Native Americans used shells and pearls for food, decoration, and tools. American colonists used the inside of mussel shells for making buttons. And, more recently, freshwater mussels have been used to obtain pearls.

Freshwater Mussels Need Our Help!

Freshwater mussels are the most imperiled group of organisms, at a greater risk of extinction than birds, mammals, and reptiles combined!

Freshwater mussels are threatened by:

  • Habitat disturbance and loss

  • Pollution

  • Disruption of reproduction

  • Competition with non-native, invasive species

Environmental contaminants and toxins in stormwater runoff, heavy nutrient loading from agricultural sources and fertilized lawns, large increases in temperature (thermal pollution), increased erosion and siltation (due to loss of riparian buffers, construction activities, and some agricultural practices), and construction of dams and impoundments all pose a threat to freshwater mussels.

Furthermore, freshwater mussels have a very unique life cycle, which can be easily disrupted by environmental disturbances. Fish serve as hosts for larval mussels and must be present in the same habitat as freshwater mussels for most species to produce young. Oftentimes, certain species of fish are required for this life stage to occur. Environmental conditions also have to be optimal for female mussels to release glochidia (larvae). An interruption of any one of these stages can have an affect on local mussel populations.

An emerging threat to freshwater mussels are non-native species . Non-native zebra mussels can smother and overcome native species. Non-native fish either eat the mussels or displace or consume host fish.

Monitoring Activities

Fourteen out of the sixteen native species of freshwater mussels known in Maryland were historically located in Montgomery County. Current distribution data is lacking. Collection of this data is the first step needed to conserve the remaining mussel populations and maintain high quality water and habitat.

Likewise, collection of distribution data for non-native species, such as the Asian Clam (Corbicula), will help determine the effects of introduced species on the environment.

DEP began monitoring freshwater mussels in the summer of 2007. As part of DEP's biological monitoring efforts for aquatic insects and fish, biologists now also look for freshwater mussels and Corbicula. The presence of dead shells or live individuals is recorded.

Mussels and clams can be encountered while sampling for aquatic insects, netting fish, and evaluating stream habitat. Biologists also search the shoreline and stream bottom for mussels and clams while searching for amphibians and reptiles. Special attention is paid to animal middens. Muskrats prey upon mussels and leave shells in piles. These middens can be found on streambanks, in shallow water, or under cover such as fallen logs.

With the exception of Corbicula, dead shells are collected and saved to verify the distribution record. No live mussels are removed or harmed.

DEP is also investigating the presence of freshwater mussels in stormwater ponds, lakes, and impoundments.

DEP can use the distribution data as part of ecological assessments and Maryland National Capital Park and Planning can use this information during land acquisition for parkland and land use planning. DEP shares distribution information with the Maryland Department of Natural Resources Natural Heritage Program and they may use DEP's information to conduct additional surveys and studies, and in management decisions.