Description and potential sources of a shell deformity in North American freshwater mussels (Unionoida).

Objective: Freshwater mussels of the order Unionoida are among the most imperiled taxa in North America, and many species are undergoing enigmatic decline without fully understood causation. Disease pathology and parasitology have been identified as areas with significant knowledge gaps in relation to these declines. We investigated a shell deformity of unknown cause that is widespread in northeastern North America by adding to the clinical description from a mussel assemblage in Massachusetts with a deformity prevalence exceeding 50%.