Microplastic Particle Size Effects in Fish and Shellfish: A Review on Feeding, Digestion, Bioaccumulation, and Seafood Safety Risks

Microplastics (MPs) have become a pervasive global contaminant, entering marine ecosystems, food webs, and ultimately the human diet. Their environmental and biological effects are strongly size-dependent, influencing ingestion, retention, tissue penetration, and bioaccumulation. Nanoplastics (<1 μm) are of particular concern because they can cross epithelial barriers and accumulate in internal organs, while larger particles are generally confined to the gastrointestinal tract, where they cause physical abrasion and localized tissue injury. Fish and shellfish are critical study targets due to their ecological roles as filter feeders, benthic deposit feeders, and predators, as well as their direct role in transferring MPs from the marine environment to human consumers. Evidence shows that particle size regulates digestive tract damage, distribution in tissues such as gills, liver, and muscle, and trophic transfer efficiency within marine food webs. Nonetheless, significant gaps persist, including inconsistent classification of particle size ranges, limited attention to particles smaller than 10 μm, and inadequate integration of ecotoxicological findings with food safety risk assessments. This review synthesizes current knowledge on the size-dependent effects of MPs in fish and shellfish, with a focus on implications for seafood safety and human health. Advancing standardized methodologies and incorporating size-based risk into monitoring and regulation are urgently needed to guide sustainable fisheries management and protect public health.