Parasites of QX-resistant and wild-type Sydney rock oysters (Saccostrea glomerata) in Moreton Bay, SE Queensland, Australia: diversity and host response.

Wild caught (WC) and QX resistant (QXR) Sydney rock oysters were introduced at North Stradbroke Island and Pimpama River, SE Queensland, Australia, and sampled monthly during 1 year. Three groups of parasites/diseases were identified by observation of histological sections: (1) Marteilia sydneyi (Queensland unknown (QX) disease) and Steinhausia sp. (Microsporidia) characterized by a high prevalence and deleterious impact on the host; (2) disseminated neoplasia and the trematode Proctoeces sp. characterized by low prevalence but deleterious effects on the host; (3) parasites or symbionts with no detectable effect on the host: trematodes, ciliates, turbellarians and metacestodes. Mortality rates were similar between both oyster lines but higher at Pimpama River (reaching around 90%) than Stradbroke Island, mostly because of QX disease and, to a lesser extent, to the unfavourable environmental conditions of the summer 2010-2011. Lower prevalences of QX disease at Stradbroke Island probably related to the relative lack of intermediate hosts of the parasite and to lower freshwater input. Surprisingly, no difference in prevalence of QX disease was observed between the two oyster lines.