Heavy metals are the most important pollutants in aquatic ecosystems that may cause adverse effects on its biota. In this study, histopathological effects of zinc (Zn) and their incidence time on swan mussel, Anodonta cygnea, were studied. Exposure to Zn was done during 18 days and histopathological investigations were conducted in mantle, digestive gland and foot in days 0, 4, 9, and 18. Histopathological changes observed were: damages of epithelium cells with increasing mucous cells (in mantle), atrophy of digestive tubules and haemocyte aggregation (in digestive gland), and hypoplasia, increasing mucous cells and myocyte swelling (in foot). Moreover, granuloma and tissue rupture were found in all organs. Primary histopathological changes were observed in fourth day of examination in all of studied organs. Results showed that sensitivity of digestive gland is lesser than mantle and foot in exposure to Zn. Also the results indicated the histopathological alterations in the organs of swan mussel can be considered as reliable biomarkers in biomonitoring of heavy metal pollution in aquatic ecosystems
