The fanshell (Cyprogenia stegaria) is a species of aquatic bivalve mollusk in the family Unionidae.

This clam is native to the United States, where breeding populations remain in only three rivers.

It is a federally listed endangered species of the United States.

[3] All Unionidae are known to use the gills, fins, or skin of a host fish for nutrients during the larval glochidia stage.

Cyprogenia stegaria release into the water a conglutinate mimicking an Oligochaeta worm which contains the mussel's young.