Evaluating the influence of polymer identity versus particle abundance on microplastic pollution indices in brachyuran crabs

Coastal ecosystems face escalating contamination from microplastics, yet related assessments frequently rely on particle abundance while neglecting polymer-specific toxicity—a critical oversight given orders-of-magnitude differences in hazard potential among plastic types. We conducted a field-based assessment and evaluate how polymer identity versus particle load of microplastic influences ecological risk in three brachyuran crab species ( Portunus pelagicus, Baptozius vinosus, Scylla olivacea ) inhabiting mangrove habitats across Cebu, Philippines. Gastrointestinal tracts and gills underwent alkaline digestion, density separation, vacuum filtration, and polymer identification via ATR-FTIR spectroscopy, with hazard scores assigned according to monomer toxicity rankings. Microplastics occurred ubiquitously across all specimens, predominantly as transparent and white fibers (57%). B. vinosus exhibited the highest particle density (2.98 ± 0.90 MPs/g) but moderate hazard (Hᵢ = 539.38) owing to polypropylene dominance (48.6% of particles; hazard score = 1), whereas S. olivacea displayed the lowest density (0.41 ± 0.09 MPs/g) yet highest hazard (Hᵢ = 2,452.66) mostly due to accumulation of polyacrylamide (hazard score = 22,240). Spearman correlations (ρ) between contamination factor were non-significant for two of three species ( P. pelagicus: ρ = 0.066, p = 0.730; B. vinosus: ρ = 0.273, p = 0.159), with only S. olivacea showing moderate significant relationship (ρ = 0.503, p = 0.009)—demonstrating that polymer composition, not particle count, governs ecological hazard in these mangrove-associated crustaceans. Studies that quantify microplastics based solely on particle counts, or even those that identify polymers without evaluating their inherent hazard properties, will potentially fail to capture the true toxicological implications of contamination. Microplastics assessment must always therefore incorporate polymer hazard evaluation to generate ecologically meaningful interpretations of microplastic pollution especially among coastal biota.