Pennsylvania Field Office
Northeast Region

Life History and Biology

rayed bean Plethobasus cyphyus

STATUS: Proposed Endangered

DESCRIPTION: The sheepnose is a medium-sized mussel that reaches nearly 5.5 inches in length. The shape of the shell is elongate ovate and moderately inflated with thick solid valves. There is a row of large broad bumps on the center of the shell. The shell surface is generally smooth, shiny, rayless, and light yellow to a dull yellowish brown. Concentric ridges resulting from rest periods are usually darker. Key characteristics useful for distinguishing the sheepnose from other mussels are its yellow shell color, the occurrence of central tubercles, and its outline.

RANGE: Historically, the sheepnose occurred throughout much of the Mississippi River system with the exception of the upper Missouri River system and most lowland tributaries in the lower Mississippi River system in Minnesota, Wisconsin, Iowa, Illinois, Missouri, Ohio, West Virginia, Indiana, Kansas, Kentucky, Tennessee, Alabama, Mississippi, Pennsylvania, and Virginia. The sheepnose has been eliminated from two-thirds of the total number of streams from which it was historically known (26 streams currently compared to 77 streams historically), including hundreds of miles of major rivers.

In Pennsylvania the sheepnose occurs in limited areas of the middle Allegheny River.

HABITAT: The sheepnose is primarily a larger-stream species often occurring in shallow shoal habitats with moderate to swift currents over coarse sand and gravel but may also have mud, cobble, and boulder substrates. Specimens in larger rivers may occur in deep runs.

Sheepnose are sedentary filter feeders, obtaining oxygen and food directly from the water column or from water flowing through the substrate. Reproduction requires a stable, undisturbed habitat and a sufficient population of sauger (Sander canadensis), the only indentified host fish for sheepnose, to complete larval development. Sheepnose are relatively long-lived (30 to 50 years).

REASONS FOR CURRENT STATUS: The decline of sheepnose in many large rivers was likely the result of poor water quality and impoundment for navigation and power production in the early 1900’s. Threats to remaining populations stem from stream and land uses that reduce habitat stability and water quality such as instream gravel mining, chemical and nutrient contamination, mineral extraction, exotic species introduction, and sedimentation. Remaining populations are isolated and may be eliminated by single catastrophic events, such as toxic spills. Natural repopulation is impossible without human intervention.

Conservation actions that may benefit sheepnose are programs that support life history research and surveys and those that contribute to public understanding of the functions that sheepnose and other mussels play in the environment. Ensuring that regulations designed to protect water quality and aquatic habitats are fully implemented is vital to maintaining or enhancing sheepnose populations.

Proposed Listing Rule
Sheepnose Fact Sheet
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Species Information
Last updated: May 4, 2011
All images by FWS unless otherwise noted.