To investigate a unionid mussel die-off that began in 2016 in the Clinch River, organ tissues from 10 pheasantshells Actinonaias pectorosa (Conrad, 1834) were collected from Kyles Ford, TN, on October 6, 2016. Histological evaluations quantified abundances of tissue lesions in gills, digestive glands, and kidneys; gamete production and resorption levels in reproductive acini; and percentages of mussels infested with parasites. Atrophy and necrosis of digestive gland tissues were prevalent in most mussels, and pervasive necrosis in vesicular connective tissues (VCT) also was observed. Nine of the 10 mussels were infested with parasites. One mussel stained Gram positive, and examination revealed the presence of unidentified streptococci-like bacteria. Nine mussels stained Gram negative, and the stained spherical structures resembled cocci. High fractions of kidney cells containing lipofuscin were observed in the study mussels, indicating high levels of oxidative stress and probable impairment of important organ functions. It is likely that oxidative stress and consequent lipofuscin abundances in the kidney cells resulted from the combined effects of lesions (tissue damage, parasitic infestations, and presumptive bacterial infections) and the low-concentration contaminant array in the Clinch River. Necrosis in digestive glands and VCT decreased the available somatic energy (i.e., lipids and carbohydrates), although energy demands were high because of parasitic infestation, presumptive bacterial infection, altered physiological functions, and needed tissue repair. Although the results of this study could not determine the actual causes of mortality during the mussel die-off, they revealed numerous pervasive tissue lesions in the mussels, which led to starvation-like conditions due to unidentified environmental stressors.
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Vol. 38 • No. 3