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Swan mussel, Anodonta cygnea, is a freshwater bivalve inhabitant of Anzali lagoon. Its valuable feature is recycling nutrients and improving water clarity. In recent years, their population in the Anzali lagoon is in danger due to pollution and habitat destruction. Therefore, more studies on biological characteristics of these animals are very important for future conservation and management actions. In the present study, the reproductive cycles and gametogenesis of A. cygnea were evaluated in a one-year period (on a monthly basis) using fresh smear and histological study by standard method. In all shells, two sexes were separated. There was no apparent sexual appearance. Gametogenesis has been continuous throughout the year. It was activated in the spring, and peaked in the summer and early autumn. The final maturation, spawning and spermiation occurred in September. Glochidia were present in gills in January, and in March, gills were completely empty of Glochidia. Spermatogonia, spermatocyte, spermatid and spermatozoa were found in male gonad acini. The sexual pattern was asynchronous due to simultaneous presence of different stages of oogenesis and spermatogenesis in gonadal acini. In both males and females, there were found three gonadal phases during a maturation period: developing stage, spawning, and post spawning. They were in developing phase in spring (May and June) and in summer (July and Agust). The spawning stage occurred in the autumn and the post-spawning stage in the winter. Although, gametogenesis was active in all seasons, the peak of oogenesis was observed in July and September and the peak of spermatogenesis was in September. The development of Gloxidia was completed in January and the release of them from the gill has been occurred between January and March. Keywords: Anodonta, Anzali lagoon, Bivalve, Gametogenesis, Maturation
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