CHAETOGASTER-LIMNAEI (ANNELIDA, OLIGOCHAETA) AS A PARASITE OF THE ZEBRA MUSSEL DREISSENA-POLYMORPHA, AND THE QUAGGA MUSSEL DREISSENA-BUGENSIS (MOLLUSCA, BIVALVIA)

Dreissenid mussels, Dreissena polymorpha and D. bugensis, were found t o be infected by the naidid oligochaete Chaetogaster limnaei at four s ites in the St. Lawrence River. This is the first report of this speci es infecting dreissenids anywhere in the world. Most worms inhabited t he mantle cavity, where they caused erosion of the mantle and gill epi thelia as determined by histopathological examination. Others penetrat ed various tissues; one had invaded the ovary and was feeding on oocyt es and ovarian tissues. Of 606 mussels examined, 166 (27.4%) harbored at least 1 C. limnaei. The prevalence varied between 1% and 80%, depen ding on the collection site and date. The worms were slightly but sign ificantly more prevalent in D. bugensis than in D. polymorpha. The int ensity ranged from 1 to 18 worms per infected host. Variations in prev alence and intensity were not related to the size or sex of the host, but the data did suggest some seasonality.