Histopathological and Physiological Impacts of Microplastics on Freshwater Fish Gill Morphology and Respiration in Indian River Systems: A Critical Evidence Synthesis and Systematic-Review Framework

Abstract

Background: The fish gill is simultaneously a respiratory, ionoregulatory, acid–base, excretory and immunological interface. Microplastic particles have been recovered from Indian freshwater environments and fish tissues, but detection of particles is not equivalent to demonstration of gill injury or impaired oxygen uptake. Objective: To critically synthesize evidence concerning microplastic-associated gill lesions and respiratory outcomes, determine which findings arise specifically from Indian river systems, and develop a reproducible protocol for a formal systematic review. Approach: A structured, source-verified evidence synthesis integrates Indian observational studies with mechanistic freshwater-fish experiments and methodological guidance. Findings: Investigations from the Ganga and northeast/southern Indian rivers establish exposure and organ-specific particle occurrence. Controlled fish studies report epithelial lifting, lamellar fusion, vascular remodelling, mucus-cell proliferation, oxidative injury, inflammatory signalling and altered respiration; however, findings differ by species, particle size, exposure dose and study design. A 28-day tilapia experiment using 80 nm, 2 µm and 20 µm polystyrene at 100 µg/L found different respiratory impacts by size, illustrating the importance of particle-specific inference. Interpretation: Mixed river pollution, inconsistent particle confirmation, limited paired physiology and histology, and incomplete exposure characterization currently constrain microplastic-specific causal attribution in Indian rivers. Conclusion: A tiered design integrating FTIR/Raman-confirmed particles, blinded quantitative branchial histopathology, respirometry, water-quality covariates and well-controlled exposure experiments is needed before population-level respiratory risk can be reliably estimated.