Ecotoxicological Implications of Microplastics on Aquatic Vertebrates: Disruption of Homeostasis and Regulatory Pathways

Microplastics (MPs) and nanoplastics (NPs) have become ubiquitous contaminants across global aquatic environments, posing severe risks to aquatic vertebrates which serve as critical sentinel species for ecosystem health and human food security. This review synthesizes current literature (2015–2025) to move beyond simple ingestion documentation and analyze the sophisticated mechanisms by which MPs induce systemic homeostatic failure. The analysis highlights that toxicity is size-dependent, with nanoplastics (<100 nm) presenting enhanced risks due to superior mobility and sorption capacities. Key mechanisms identified include the "vector effect," where MPs transfer co-contaminants like endocrine-disrupting chemicals (EDCs) and heavy metals to biological tissues, significantly altering their bioavailability. This exposure triggers a cascade of physiological disruptions, primarily targeting the neuroendocrine system (HPG, HPT, and HPA axes), leading to reproductive toxicity and metabolic deregulation. Furthermore, MPs induce oxidative stress, immunotoxicity, and gut dysbiosis, which collectively compromise organismal fitness and survival. The review concludes that MP exposure represents a chronic, multi-modal assault on aquatic life, necessitating standardized risk assessment protocols and urgent policy interventions to mitigate these profound ecological impacts.