Understanding global microplastic pollution across terrestrial and aquatic ecosystems through insect-fish comparative insights

• Microplastics ingestion is driven by complex biological and environmental factors • Fish and insects share core microplastic toxicity pathways despite distinct biology • Some effects appear taxon-specific, reflecting true and study-driven divergences • Microplastics in fish and insects disrupt food webs and ecosystem services • Fish and insects act as key biological vectors of microplastics across ecosystems Microplastics (MPs) contaminate terrestrial, freshwater, and marine ecosystems worldwide, yet the mechanisms linking their ingestion, biological effects, and ecological redistribution by organisms remain poorly integrated across taxa and environments. Although many organisms ingest MPs, existing evidence is often fragmented by ecosystem or species group, limiting our ability to identify broader patterns. This review addresses this gap by examining fish and insects, two ecologically distinct and influential groups that collectively span all major ecosystems, to reveal cross-taxon insights in MP exposure and impacts. We synthesize current knowledge on MP sources, environmental distribution, and diversity, and compare the mechanistic pathways through which organisms are exposed to MPs and how such exposure affects physiology, behavior, development, reproduction, and gut microbiota. Despite their contrasting anatomies and life histories, fish and insects exhibit convergent responses to MPs and play key roles in their redistribution through trophic transfer, movement, and cross-ecosystem life cycles. Some species from both groups demonstrated the ability to alter or degrade polymers, likely mediated by their microbiota, with potential implications for MP fate. This cross-taxon perspective clarifies how individual-level effects scale to ecosystem processes and highlights uneven research efforts across taxa, which hinder accurate comparisons. This underscores the need for harmonized, standardized, and ecologically realistic approaches to advance global assessments of MP pollution.