The Fanshell (Cyprogenia stegaria) is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels. Historically widespread throughout the Ohio, Cumberland, and Tennessee river systems, this species, native to the United States, now occurs in only a few isolated populations and faces a severe threat to its continued existence. It is federally listed as an Endangered species under the U.S. Endangered Species Act (ESA) due to a dramatic and ongoing decline in its populations and a significant contraction of its historical range. The International Union for Conservation of Nature (IUCN) lists it as Critically Endangered. As a filter-feeding organism, Cyprogenia stegaria plays an integral role in its riverine ecosystems, contributing to water clarity and nutrient cycling. Its decline, therefore, represents not only the potential loss of a unique species but also a diminishment of these vital ecological functions.

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