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We assessed the concentration of organophosphate pesticides (OPs) and microplastics (MPs) in water and sediments from the burrows, and tissues of the fiddler crabs Leptuca festae and Minuca ecuadoriensis, from Isla Santay, a Ramsar site in the estuary of the Guayas River, Ecuador. MPs concentrations in the burrows were 660 ± 174.36 items kg−1 (w.w.) and 26 ± 1 items L−1 in collected sediments and water, respectively. Regarding OPs, water and sediment concentrations were up to 26 times above the USEPA thresholds for chronic exposure, indicating environmental risk. MPs were found in tissues collected from both species. The highest abundance was in the gills followed by the digestive tract and hepatopancreas. OPs concentrations in tissues were below the detection limits. Because fiddler crabs are chronically exposed to environmental contamination, they are suitable bioindicators to monitor Isla Santay and to comprehend human impacts in coastal environments of Ecuador.

We assessed the combined effects of polyethylene microplastic (MP) and malathion (MLT) on the survival of the fiddler crab Minuca ecuadoriensis, and MP tissue bioaccumulation in four treatments following 120 h exposure: T1) Control; T2) MLT 50 mg L− 1 ; T3) MP 200 mg L− 1 ; and T4) MLT (50 mg L− 1 ) + MP (200 mg L− 1 ). The highest mortality (80%) was in T4, followed by T2 (28%) and no mortality was in T3. Higher MP bioaccumulation was observed in T4 (572 items g tissue− 1 ) followed by T3 (70 items g tissue− 1 ). Our findings indicate that the synergistic effect of MLT and MP increased M. ecuadoriensis bioaccumulative capacity and decreases survival. Thus, as MP contamination in aquatic environments is ubiquitous, our study raises a warning on the synergistic effects of MP with other environmental contaminants and serves as a baseline for further studies.