Freshwater mussels (Unionidae) are one of the most endangered taxonomic groups in the United States. Approximately 12 species of unionids have been documented as residing in the Susquehanna River.
Elliptio complanata was once the dominant species of mussel in the Susquehanna drainage but in recent years has been shown to have a declining population. Based on size distribution by species, we found the eastern elliptio (Elliptio complanata) is suffering from a loss of recruitment. One major factor behind this lack of young E. complanata may be the loss of the American eel from the upper Susquehanna River basin due to damming. Unionid freshwater mussels have a unique interaction with fish. Many use fish as a host for their glochidia or in their larval stage. Some mussel species are functionally dependant on a specific host fish to complete its life cycle. Mussels without the specific host fish interaction face complications with reproduction and a diminishing population and eventual extirpation or extinction. American eels are the primary host fish of larval (glochidia) E. complanata. While this species has been documented using a number of other host fish species (Kneeland and Rhymer 2008), our data suggests that this is limited in the West Branch of the Susquehanna River.