Histopathologic survey of free-living populations of 2 species of freshwater mussels in Indiana.

Freshwater mussels are one of the most endangered groups of animals in Indiana, with nearly half of the native species either extirpated or listed as "state endangered" or of "special concern." Nationally, numerous freshwater mussel species are considered threatened. Freshwater mussel diseases are not well understood and few published accounts of freshwater mussel diseases with detailed histological descriptions exist. Mass mortality events within mussel populations are increasingly recognized, often with undetermined etiology. Our objective was to determine baseline histopathology in free-living populations of freshwater mussels. One-hundred twenty individual mussels representing 2 species-plain pocketbook () and fatmucket ()-were collected from 3 different locations within the Wildcat Creek watershed in central Indiana during June and July 2019. A cross-section through the visceral mass was obtained and immersed in 10% neutral-buffered formalin, with routine processing and hematoxylin and eosin staining. Branchial acariasis occurred in 43/60 fatmuckets and 22/60 plain pocketbooks. Infection with a bucephalid trematode was recognized in 18/60 fatmuckets, while infection of the gonadal duct with an unidentified trematode species was identified in 4/60 fatmuckets and 18/59 plain pocketbooks. Additional changes associated with unidentified trematodes, bacteria, fungi or oomycetes, and ciliates were observed. Other miscellaneous changes included mineralization, neuronal lipofuscinosis, and gonadal atrophy/atresia. A range of histological changes were observed. These changes likely represented background lesions: incidental findings, spontaneous infectious or endosymbiotic conditions, or normal physiological changes that routinely occur in free-living wild populations. Awareness of baseline lesions should inform future diagnostic investigations of mussel mortality events.