Emerging microplastic threats to horseshoe crabs: accumulation patterns and toxicity mechanisms

Micro- and nanoplastics possess serious ecotoxicological threats to ancient lineages like horseshoe crabs. This review comprehensively synthesize MP occurrence, ecological risks, influencing factors and underlying toxicity mechanisms in horseshoe crabs while identifying knowledge gaps for future endeavors. MPs have been reported in gastrointestinal tracts of T. tridentatus (11.8 MPs/ind.) and eggs of L. polyphemus (633 MPs/g), with no data available for C. rotundicauda and T. gigas . Most studies reported fiber-shaped MPs (60%) and 1–5 mm sized MPs (37.5%) in horseshoe crabs, with polyamide, polypropylene, and cellophane were most commonly observed polymer. Detected polymers pose moderate-to-high ecological risks and may accumulate from costal sediment, water, and contaminated prey. Furthermore, MP/NPs alone or co-exposed with heavy metal (Cu²⁺) and norfloxacin, induce growth, behavioral, metabolic, oxidative, gut-microbiota, digestive enzyme alterations in T. tridentatus . Finally, future research on source identification, tissue-level accumulation, and immuno-endocrine disruption in horseshoe crabs, together with improved coastal pollution management is recommended.