Abbie Gascho Landis first fell for freshwater mussels while submerged in an Alabama creek, her pregnant belly squeezed into a wetsuit. After an hour of fruitless scanning, a mussel materialized from the rocks--a little spectaclecase, herself pregnant, filtering the river water through a delicate body while her gills bulged with offspring. In that moment of connection, Landis became a mussel groupie, obsessed with learning more about the creatures' hidden lives. She isn't the only fanatic; the shy mollusks, so vital to the health of rivers around the world, have a way of inspiring unusual...