Abstract
<title>Abstract</title> <p> Contemporary pollution issues are primarily focused on Contaminants of Emerging Concern (CECs), including microplastics such as tire wear particles (TWPs) and textile microfibers (MFs). These tiny particles can be ingested by aquatic organisms and bioaccumulate, yet interactions with other coexisting contaminants remain unclear. This study evaluated the toxicity of MFs, TWPs, and their mixtures with the pyrethroid insecticide bifenthrin to the estuarine grass shrimp ( <italic>Palaemonetes pugio</italic> ) using 96-hour acute, static renewal toxicity tests. Survival was assessed under standard conditions (20°C, 20 psus). No mortality occurred in shrimp exposed solely to MFs or TWPs at the highest tested doses. Bifenthrin alone exhibited a 96-hour LC50 of 12.92 ng/L (95% CI = 3.01–24.03 ng/L). Co-exposure of bifenthrin mixtures with TWPs yielded a 96-hour LC50 of 18.14 ng/L (95% CI = 15.39–22.69 ng/L), while mixtures with MFs resulted in 19.69 ng/L (95% CI = 15.11–25.43 ng/L). Although 96-hour LC50 values for mixtures did not differ significantly from bifenthrin alone, both MFs and TWPs reduced bifenthrin toxicity over time, evidenced by higher NOEC and LOEC values after 96h of exposure. However, during the first 24 hours of exposure, TWP increased the onset of toxicity of bifenthrin in grass shrimp as evidenced, by the lowering of the NOEC and LOEC, which is most important as this would be part of the initial “first flush” associated with runoff of bifenthrin following significant precipitation. Histological analysis revealed MFs clogged shrimp gills, potentially altering bifenthrin uptake. Behaviorally, the bifenthrin + MF mixture induced hyperactivity (e.g., head/rostrum bumping), absent in bifenthrin or bifenthrin + TWP exposures, likely increasing predation risk. These sublethal effects warrant further investigation in future bifenthrin-CEC interaction studies. </p>