The effects of the trematode Bucephalus polymorphus on the reproductive cycle of the zebra mussel Dreissena polymorpha in the Drava River

The effects of the trematode Bucephalus polymorphus on the reproductive cycle of the zebra mussel Dreissena polymorpha were examined in mussel populations from the Drava River. The reproductive cycle was studied by histological examination of the gonads and quantified by an image analysing system to determine changes in volume of the entire visceral mass, gonads, digestive glands and in particular the volume of trematodes. Results confirmed that (1) gonads of D. polymorpha were affected by B. polymorphus infection more than any other organ and (2) development of cercariae in sporocysts of B. polymorphus coincides with host gonad maturation. This is the first study in which the image analysing system was used to determine the effect of trematodes on the reproductive cycle of D. polymorpha . Also, this is the first record of sporocysts of B. polymorphus in D. polymorpha in this part of Europe.