HEALTH ASSESSMENT OF FLAT OYSTERS IN NORTHERN ADRIATIC SEA: ONE YEAR MONITORING

Health monitoring of Ostrea edulis in the Northern Adriatic Sea was conducted as a part of the INTERREG project MARINET, which aims to evaluate the feasibility of an Integrated Multi-Trophic Aquaculture (IMTA) model involving flat oysters and European sea bass. Juvenile flat oysters from Lim Bay (Croatia) were placed in lantern nets near sea bass farms in Duino (Italy). Health assessment were conducted for endemic diseases (Bonamia spp., Marteilia refringens) and other potential pathogens. 40 oysters were sampled every two months over a one-year period and analyzed following EURL protocols (IFREMER). Histological analysis revealed a widespread presence of parasites referable to Perkinsus genus in oyster samples from Duino. The species was confirmed as Perkinsus mediterraneus by molecular analysis of the internal transcribed spacer (ITS) region of the ribosomal RNA gene complex. Schizonts and trophozoites of P. mediterraneus were observed in the connective tissues of the visceral mass, gonads, gills, labial palps, and mantle, often associated with significant haemocyte infiltration. Its prevalence, assessed by histology, varied during the monitoring period (2.5% - 85%) with a peak observed in October (17.5°C). In addition to P. mediterraneus, Bonamia exitiosa was detected by real-time PCR from June to October but was not recorded from December to February, with temperatures below 15°C. A co-infection of B. exitiosa and P. mediterraneus was also identified in one specimen. Three oysters (p=1.25%) exhibited disseminated neoplasia. Neoplastic cells, characterized by reduced cytoplasm, enlarged nuclei, and the presence of mitotic figures, were observed in the connective tissues of various organs and in the haemolymph. This is the first report of P. mediterraneus in Northern Adriatic, with temperature appearing to influence parasite prevalence. The detection of Perkinsus mediterraneus, Bonamia exitiosa and disseminated neoplasia in farmed Ostrea edulis highlights the importance of health monitoring.