The Upper Mississippi basin is a global hotspot for freshwater mussels, with over 300 species. That's more than half the world's total. Unfortunately, these filter-feeders depend on clear, unpolluted waters -- which are all too rare in this heavily industrialized area. Dozens of species are listed as threatened or endangered, and only a lack of scientific information keeps more of them off the list.
Against this background, seeing lots of mussels is one of the joys of hiking the Ozarks. It means the stream is healthy.
These are obviously little ones, imaged in January 2011.