FWS Focus



?A native freshwater mussel species of fast-flowing riffles in medium-sized rivers, the winged mapleleaf leads a complex life for an animal sometimes confused for a rock. Like many other freshwater mussels, this endangered species gets its name from its shape, and how it varies from its far more commonly found sister taxon the mapleleaf, a wing or protrusion along one side. Winged mapleleaf were once found in 38 places in the midwest from Minnesota to Arkansas, but are now found only in six.

The winged mapleleaf, like other species of mussels, spends most of its time buried in sediments and are primarily sedentary. To move, they use a hatchet-shaped muscle called a foot which contracts and allows the mussel to pull itself along. The St. Croix River in Minnesota contains the only populations known to be reproducing.

The winged mapleleaf is currently listed as an endangered species. Two of the main threats to the winged mapleleaf are habitat fragmentation and small population size. For example, the five remaining populations are largely or entirely isolated from one another and appear to be highly differentiated genetically, as noted by A.H. Hemmingsen in 2008. Thus, each may possess important local adaptations. Propagation of winged mapleleaf, and its reintroduction into historically occupied river reaches, could reduce the impact to the species’ genetic diversity that might result from the loss of any of the remaining populations. In addition, it may be an important tool to increase the viability of one or more populations. Along with several partner agencies, we are working to establish a successful protocol for producing winged mapleleaf in large enough numbers to effectively reintroduce the species.

Zebra mussels also (Dreissena polymorpha) impact the remaining populations of winged mapleleaf. Zebra mussels are an invasive species invasive species
An invasive species is any plant or animal that has spread or been introduced into a new area where they are, or could, cause harm to the environment, economy, or human, animal, or plant health. Their unwelcome presence can destroy ecosystems and cost millions of dollars.

Learn more about invasive species
of mussel that threaten native freshwater mussels in the Mississippi River Watershed. They were inadvertently introduced into North America during the late 1980s and became established in the lower St. Croix River in 2000. Zebra mussels attach to any hard surface and breed so prolifically that they smother or otherwise harm native mussels. It is essential to the conservation of winged mapleleaf that zebra mussels are not allowed to invade any of the remaining winged mapleleaf habitat.

The St. Croix River population is near the major metropolitan area of Minneapolis and St. Paul in Minnesota. Therefore, major land use changes like urban development of the St. Croix River Watershed could result in increasing levels of contaminants and sediments in runoff that drains into the river. In 2008, D.J. Hornbach and M.C. Hove documented an increase in fine sediments in the winged mapleleaf habitat in the river. In Arkansas, Missouri and Oklahoma, agriculture and industry are abundant in the watersheds where winged mapleleaf are present. These activities can destabilize river corridors and increase runoff of harmful pesticides, chemicals and sediment.

On July 22, 1991, the species was added to the U.S. List of Endangered and Threatened Wildlife and Plants. This status makes it illegal to harm, harass, collect or kill the mussel. Scientific studies of winged mapleleaf or forms of take that is incidental to an otherwise legal activity may be allowed by obtaining a permit from the U.S. Fish and Wildlife Service. In addition to listing the species, U.S. Fish and Wildlife Service researchers developed a recovery plan to identify and prioritize actions necessary to recover this species.

Lastly, we have been working with Xcel Energy, which operates a dam upstream of the winged mapleleaf habitat in the St. Croix River to protect needed habitat. The energy company agreed with Wisconsin Department of Natural Resources to manage flows from its St. Croix Falls dam in a run-of-river mode. This effectively ended peaking operations at this facility that adversely impacted the important mussel beds just downstream. In addition, the National Park Service, the states of Minnesota and Wisconsin, the U.S. Army Corps of Engineers and others are cooperating with us to collectively keep zebra mussels out of habitat which is occupied by winged mapleleaf in the St. Croix River.

Scientific Name

Quadrula fragosa
Common Name
winged mapleleaf
winged mapleleaf mussel
FWS Category

Location in Taxonomic Tree

Identification Numbers



Characteristic category

Physical Characteristics

Size & Shape

Adults can grow to be up to four inches in length.  

Color & Pattern

Winged mapleleaf have thick shells that are greenish brown, chestnut or dark brown in color. Their shell, like that of a few other native freshwater mussel species, has several rows of bumps running from the hinge, or umbo, to the edge of the shell. The patterns of these rows of bumps, or tubercles, help biologists differentiate this from other, similar mussel species. Faint rays are visible in small shells. The epidermis of adults is dull brown, usually with two or three broad and widely interrupted green rays, as described by T.A. Conrad in 1835 and later by Simpson in 1914 and Ortmann in 1924.

Characteristic category

Life Cycle

Life Span

The life span of the winged mapleleaf is not known, but the oldest known individual in the St. Croix is 22 years old.


Winged mapleleaf reproduction is similar to many other freshwater mussels. The males shed sperm into the water. Eggs on the gills of females are fertilized when sperm is collected as the female siphons in water. After fertilization, the females store the developing larvae, known as glochidia, in their gills until they’re expelled into the river current. These glochidia are either spined or hooked, depending on the subfamily, as documented by S.L.H. Fuller in 1974. The glochidia must attach to the gills or fins of a fish to complete development and can only develop on certain species of fish, which are called host fish. Known host fish for the winged mapleleaf are channel and blue catfish. Glochidia continue growing on the fish and transform into juveniles, then they drop off and land on the river bottom where they mature into adults.

Characteristic category



Winged mapleleaf are found in riffles with clean gravel, sand or rubble bottoms and in clear, high quality water. In the past, it may also have been found in large rivers and streams on mud, mud-covered gravel and gravel bottoms. 

River or Stream

A natural body of running water.

Characteristic category



To feed, the winged mapleleaf siphons in water and filters out food particles. It is thought that most of the particles that are actually used as food are phyto and zooplankton – tiny organisms that drift with river currents. Detritus is also thought to form a significant fraction of its diet and may be obtained either from suspension or deposit feeding, as documented by Way in 1990.  



When the recovery plan was approved in 1997, only one population of winged mapleleaf was known to be extant – in the St. Croix River on the border between Minnesota and Wisconsin. Since then, four additional populations have been found: in the Saline River and Ouachita River in Arkansas, in the Little River in Oklahoma and in the Bourbeuse River in Missouri. Additional surveying is needed to assess and monitor the status and distribution of each population. 

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