engleski

Hermaphroditism in zebra mussel, Dreissena polymorpha (Pallas, 1771): impact of trematode parasitism?

Zebra mussel, Dreissena polymorpha (Pallas, 1771) is one of the most successful invaders of freshwater ecosystems in Europe and North America. In this study I examined the reproductive cycle of D. polymorphain in order to identify: (1) the presence of hermaphrodites in the population, (2) the presence of trematodes in mussels and (3) the relationship between the presence of trematodes and appearance of hermaphroditism. Mussels were collected from the dam reservoir Dubrava and Drava River in eight sampling occasions during one year. Histological analyses were performed using classical histological techniques. The stage of gametogenic development was described using a four-step qualitative evaluation. The surface of the whole visceral mass, gonads, digestive glands and trematodes were measured using an image analyzing system (LUCIA G 4.81). Three species of the trematodes were observed on the histological sections of the mussels: Bucephalus polymorphus, Echinoparyphium recurvatum and Aspidogaster limacoides. Sporocysts with cercariae of the B. polymorphus were located in the gonads, metacercariae of the E. recurvatum were in the visceral sack epithelium and in connective tissue while adult specimens of the A. limacoides were observed in stomach and digestive gland. Analysis of histological preparations of 160 mussels indicated only 4 hermaphrodites. Two hermaphrodites from Drava River and one from the dam reservoir Drava were infected by B. polymorphus while the other one from the dam reservoir Dubrava was healthy. Some of the follicles in the hermaphroditic mussels were "female", some "male" and some were „hermaphroditic“. There were no significant differences in gonad development and gonad stages, between hermaphrodites and other specimens. Only the volume of gonads of the infected animals was significantly lower than gonad volume in healthy animals. Finally, as the three D. polymorpha hermaphrodites were infected by trematodes, I assume that hermaphroditism was induced by the trematode infection. Unfortunately, the number of hermaphrodites was too small to prove that presumption with statistical methods and further investigation is needed.

Dreissena polymorpha; trematodes; hermaphroditism; reproductive cycle

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