Selective ingestion drives divergence between environmental and gut microplastics in riverine fishes

Microplastics (MPs) are pervasive in freshwater rivers, yet biologically relevant exposure may not be captured by environmental concentrations alone. Here, we quantified MPs in river water and in the gastrointestinal tracts of three riverine fishes (Puntius ticto, Mystus cavasius, Sperata seenghala) across five sites along the Barnoi River (Bangladesh) to test whether gut MP characteristics decouple from ambient availability. Water and fish were sampled monthly (February-April 2025). MPs were extracted by digestion, density separation, and filtration and then characterized by shape, size, and color using microscopy. Environmental MP concentrations differed significantly among sites (2.67 ± 0.33 to 51.67 ± 9.21 MP L⁻¹; Kruskal-Wallis, p < 0.05), while gut MP abundance showed no interspecific difference (H = 1.63, p = 0.443; η²H = 0.003). Negative binomial GLMs indicated that gut MP abundance varied with site but not with species identity or fish body size. In contrast, gut MP composition deviated strongly from random uptake expectations, with significant non-random patterns for shape (χ² = 254.09, p < 0.001), size (χ² = 234.87, p < 0.001), and color (χ² = 335.33, p < 0.001). Large particles (4-5 mm), foam, and fibers were enriched in fish guts relative to river water, and blue/black particles were disproportionately retained. Collectively, these findings provide field evidence that selective ingestion structures gut MP composition independently of exposure gradients, demonstrating trait-mediated modification of exposure between environmental and internalized MPs. Trait-based metrics (size, shape, color) should therefore be incorporated into freshwater MP monitoring and ecological risk assessment to better reflect biologically relevant exposure.