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Living Resources: Freshwater Mussels
Alewife floater mussels. Photo by DRBC.
Alewife floaters, a freshwater mussel.
Photo by DRBC.

Freshwater mussels are bivalve mollusks that live along, and in, the sandy bottoms of rivers, streams and lakes.

They are considered one of, if not the, most at risk animal group in the United States, with many species considered threatened or endangered by state and federal governments.

There are over a dozen species of freshwater mussels which are native to the Delaware River Basin, but their populations have greatly declined over time.

The reduced population of freshwater mussels in the Delaware River Basin is consistent with what's happening nationwide. Habitat and water quality degradation are the primary factors for declining and imperiled mussel populations.  

Freshwater Mussel Life Cycle:

Freshwater mussels have complicated reproduction strategies dependent on fish hosts.

Female mussels internally fertilize their eggs with the sperm of nearby males. Once fertilized, the larvae are released as fish pass by.

The larvae attach to the fish’s gills until they are larger. They then drop off and settle on the stream bottom; this can be quite a distance away from where the female mussel released her eggs. 

It is believed that each mussel species relies on different fish species to host their young.

Why Care about Freshwater Mussels?

They provide valuable ecosystem services by increasing water clarity, enriching habitats, and stabilizing bed erosion.

Most importantly, as filter-feeders, freshwater mussels clean the water in which they live, improving water quality.

They suck water in, trap solids (dirt, algae, and other pollutants) in their gills, and then release clean, filtered water back into the water column.

Each mussel can filter 10 gallons of water or more every day.

Mussels are sensitive to pollution and also erosion and sedimentation; they are more likely to be found in cleaner waterways. They also can live a long time - 50 years or more. 

These factors make freshwater mussels good long-term indicators of water quality and habitat conditon.

Next Steps

A recent focus on the importance of freshwater mussel populations within an ecosystem has driven many projects within the Delaware River Basin over the last decade.

Improved, coordinated monitoring, expanded studies and enhanced efforts to protect, conserve and restore mussel habitat will be needed to ensure the sustainability of the basin's freshwater mussel populations.

Surveying Mussels
DRBC staff surveying for freshwater mussels. Photo by DRBC.
DRBC staff surveying for freshwater mussels.
Photo by DRBC.

In the early 2000s, the United States Geological Survey - Northern Appalachian Research Laboratory (USGS-NARL) conducted comprehensive, qualitative mussel surveys of the Upper Delaware Scenic and Recreational River and the Delaware Water Gap National Recreation Area.

USGS: Qualitative and Quantitative Surveys of Native Freshwater mussels in the Upper and Middle Delaware River (2000-2002)

In 2013, to build on this previously-completed survey work, DRBC and USGS-NARL, partnered to conduct a study to provide a snapshot of mussel distribution and density in the Lower Delaware Scenic and Recreational River, the river section just below the Delaware Water Gap.

In the summer of 2013, the surveys were conducted over a period of seven weeks.

The surveyors found more than 25,000 mussels! While the most common species was by far the eastern elliptio, five other species were found, as well as juveniles.

Read more: DRBC Completes Lower Delaware River Mussel Survey

Eastern floater, a type of freshwater mussel. Photo by DRBC. Eastern elliptio, a type of freshwater mussel. Photo by DRBC. A yellow lampmussel. Photo by DRBC.
Eastern floater. Photo by DRBC. Eastern elliptio. Photo by DRBC. Yellow lampmussel. Photo by DRBC.
Hey Kids - Get Crafty! Make a Freshwater Mussel Hat

Links to Learn More

Delaware Wetland Monitoring and Assessment Program: Delaware's Freshwater Mussels

Fairmount Water Works: Freshwater Mussel Hatchery

Partnership for the Delaware Estuary: Freshwater Mussels

Pennsylvania Fish and Boat Commission: A Field Guide to Pennsylvania's Freshwater Mussels