Oyster as sentinels of recent microplastic contamination: Insights from a transplant experiment

Microplastic (MP) contamination is pervasive in coastal ecosystems, yet the dynamics of MP accumulation and excretion in sentinel organisms remain poorly understood. Transplantation assays provide a controlled and field-realistic approach to evaluate these processes, complementing exclusive field or experimental studies. Bivalves have a key role in coastal ecosystems and are widely recognized as effective sentinels, but transplantation-based approaches are still scarce, particularly in the Southern Hemisphere. Here, we investigated short-term (60 days) MP accumulation and depuration in oysters (Crassostrea sp.) transplanted from an aquaculture site within a protected area to the highly urbanized Santos Estuarine System, Brazil. MP concentrations increased after 30 days and peaked at 60 days, reflecting cumulative exposure under sustained anthropogenic pressure during the deployment period, which coincided with the end-of-year holiday season. In systems where fibrous microplastics dominate, short-term population surges may also rapidly increase bioavailable MPs through wastewater-mediated inputs, particularly from domestic laundry activities. A subsequent seven-day laboratory depuration significantly reduced MP levels, although complete elimination was not achieved. Polymer type, size, shape, and color profiles were consistent between native and transplanted oysters, indicating rapid integration of local contamination signals. Transplant approaches using bivalves demonstrated to be efficient to track recent MP contamination and represent a flexible, cost-effective tool for environmental monitoring in regions where native populations are absent or sparse.