Environmental drivers of pathogen and annelids interactions in oyster farming systems of a subtropical estuary

Oyster aquaculture is expanding worldwide, but the interaction between farming structures, biofouling organisms, and pathogens remains a major constraint for production efficiency and stock health. Farming systems differ in exposure, water flow, substrate availability, and biofouling pressure, potentially influencing pathogen prevalence and the development of boring annelids such as Polydora spp. In the Paranaguá Estuarine Complex (southern Brazil), oysters are produced on longlines, mud-based culture, and natural mangrove banks, yet information on how these environments mediate host–parasite interactions is limited. Here, we evaluated the occurrence of Vibrio parahaemolyticus , Perkinsus sp., and boring/epibiotic annelids in Crassostrea sp. across culture types, bays, and seasons. We quantified annelid assemblages on shells, detected V. parahaemolyticus and Perkinsus sp. in gills and rectum, and assessed oyster condition index (CI). Using multimodel inference, we tested whether pathogens and annelids predicted CI and whether farm type influenced infestation patterns. Although V. parahaemolyticus , Perkinsus spp., and Polydora spp. were widespread, none showed strong negative effects on CI. However, mud-based farms consistently harbored higher Polydora loads, indicating their potential role as reservoirs or amplifiers of shell-boring annelids under future expansion scenarios. Our results highlight the ecological importance of farming structures as substrates for biofouling communities and support the need for integrated farm management to minimize the spread of shell-boring pests in tropical estuarine aquaculture systems.