Conservation Science

Protecting Pennsylvania's Plants and Animals

Species at Risk:
Freshwater Mussels

Nestled among the rocks in the river bottom, freshwater mussels silently filter the particles from the waters of lakes, streams and rivers. They remove the silt and attached nutrients, clarifying the water and reducing particles which can cause pollution. Unfortunately, freshwater mussel populations have declined throughout Pennsylvania. WPC is working hard on the conservation of this imperiled group of invertebrates and their habitats.

Fresh Water Mussels Glit Tag
Reintroduced mussels in the Clarion River, photo by Alysha Trexler
Brook floater, freshwater mussel species

Mussels at Risk

In North America, two-thirds of the roughly 300 freshwater mussel species are imperiled, vulnerable or extinct. There are 91 mussel species across the country classified as threatened or endangered under the federal Endangered Species Act.

The same trend of declining mussel populations is occurring in Pennsylvania. Of Pennsylvania’s 67 freshwater mussel species, 10 are considered endangered in the state and another 13 have not been observed for decades. While other species are relatively widespread in Pennsylvania waterways, there are indicators that the populations may be in trouble.

For instance, some populations have limited reproduction and populations could eventually die out. This makes proactive conservation management efforts even more important to ensure long-term survival of these freshwater mussel populations.


It’s no surprise that human activities often degrade the quality of freshwater habitats in the United States. These activities often directly impact the survival of freshwater mussel populations. Impoundments, invasive species, dredging and channelization, pollution from industry, urban and agricultural runoff, absence of fish hosts and habitat destruction are all culprits to the decline in populations.

Biologists at the Pennsylvania Natural Heritage Program investigate freshwater mussel habitats throughout the state to provide information about species distributions, habitat associations, rarity and population trends that is necessary for effective management. Data from surveys and models of species habitat are used to evaluate the trends for freshwater mussel populations of conservation concern.

Power plant, Susquehanna River
Discolored water, mine pollution in the West Branch Susquehanna River
A channelized stream is one of many threats to freshwater mussel habitat.

Research and Conservation

Navigational Pools in the Ohio River

To gain an understanding of the natural resources of a large river with many human influences, WPC set out to survey the Ohio River’s freshwater mussels.

Scuba divers, Ohio River, mussel surveyThe upper river drains a 23,500 square-mile watershed in Pennsylvania, New York and West Virginia. Its basin encompasses diverse urban, agricultural and natural landscapes. Once a river of flowing riffle and runs dotted with islands, the Ohio River was converted to slow, deep pools when the navigational pools were created in the late 19th century. Since the industrial revolution, the river has received pollution from resource extraction (e.g., coal mining and timber), manufacturing (e.g., steel, glass and other materials) and storm water. Additionally, the river is also a route for the transport of goods. Barges buzz through the locks and dams, moving coal and other materials every day of the year.

Despite past and ongoing threats, the river has experienced improvements in water quality since the peak of industrial pollution. While many assume that there is little life in the deep, silty waters of the river, recent studies of fish indicate that the river is recovering.

Because mussel communities in the Ohio River are still under-surveyed, divers from the Pennsylvania Natural Heritage Program don SCUBA gear and descend to transect lines on the river bottom. Feeling for shells in the top layer of sediments and looking for the telltale shape of mussel siphons, divers pull up freshwater mussels from the sandy river bottom.

Mussel diversity, habitat preferences of species encountered and evidence of population recruitment are useful for managing the river’s resources. Information from surveys and distribution modeling about the freshwater mussel communities, particularly any discoveries of species not seen in the river in recent decades or those that prefer high-quality habitat, are also beneficial to the management of the river.

A model of species distributions in the Ohio River basin will be developed as a result of this project. The model will draw a connection between the landscape and watershed features that are typical of waterways with freshwater mussels. The results of species distribution modeling can be used for future targeted surveys and give insight into the factors related to mussel habitat.

Brook Floater and Eastern Lampmussel Distribution

To meet the need for informed management of freshwater mussels in the commonwealth, WPC biologists are updating information about the distribution of brook floater (Alasmidonta varicosa) and eastern lampmussel (Lampsilis radiata) and assessing the conservation status. The significant range decline and decrease in population sizes of the brook floater and eastern lampmussel suggest that the species are reaching a critical threshold for conservation.

Brook floater, mussel species at riskIn previous studies, PNHP biologists have failed to find brook floater and eastern lampmussel at many sites where they had occurred historically. Both species have documented declines in the habitats in the eastern United States but more information must be gathered to fully understand the extent of the habitats and threats to the mussels. In this project staff members are searching other locations that have not been visited in many years to determine what habitats remain for both species.

Over the course of this ongoing project, we are determining the gaps in distribution information and revisiting historic locations for brook floater and eastern lampmussel habitat. We will estimate population density and collect water quality and habitat information at survey locations. Models of distribution for both species will be created to better understand the relationship between landscape characteristics and each species’ habitat.

Information gathered from this project will ultimately aid in the development of species action plans and conservation or restoration projects. If necessary, the species’ conservation status will be assessed for potential state listing.

Studies in the Delaware River Basin

Eastern Pearlshell

A species that prefers cold trout streams, the eastern pearlshell (Margaritifera margaritifera) is declining across its range in northern climates in Asia, Europe and North America. WPC scientists investigated its remaining population in Pennsylvania in the Delaware River basin during a three-year study.

Eastern pearlshell, freshwater mussel

Pearlshell habitat

In the last century, the eastern pearlshell was nearly wiped out by human activities. Most of the streams in the eastern pearlshell’s home range were devastated by the acidic and metal-laden pollution discharged from anthracite coal mines. The effects of the pollution can still be seen today. Harvesting mussels for pearls during the early 20th century also decimated populations.

From 2012 to 2015, WPC conducted snorkel surveys for eastern pearlshell in the Delaware River basin. We collected water quality and stream habitat information at known eastern pearlshell locations. We also looked for previously undocumented locations, and in doing so, identified two new waterways with eastern pearlshell populations. We collected genetic samples of these mussels to analyze the population genetics and any relatedness between populations. From the information collected, we directed information to project partners about the status of eastern pearlshell and management opportunities for the species.

A federally-endangered species, the dwarf wedgemussel (Alasmidonta heterodon) is a relatively small freshwater mussel with known populations in the upper Delaware River. WPC is currently gathering baseline data to better understand the range and distribution of this species in small streams within the Delaware River basin.

This information could assist with the federal recovery of this species. Any discovery of a previously unknown dwarf wedgemussel population would be beneficial information to the management of the species. Data generated by this project helps to manage dwarf wedgemussel populations in Pennsylvania streams. This project addresses areas in need of protection in the U.S. Fish and Wildlife Service Dwarf Wedgemussel Recovery Plan. These data will be important in developing a state species assessment plan, as well as updating the federal recovery plan.

Dwarf wedge mussel, freshwater mussel species

Dwarf wedgemussel

More Species at Risk