Instar-Dependent Molecular Response Strategies to Microplastic Exposure in Endangered Juvenile Horseshoe Crab Tachypleus tridentatus

Abstract

Microplastic pollution has become an emerging threat to marine biodiversity, yet the developmental-stage-specific responses of endangered marine species remain poorly characterized. Tri-spine horseshoe crab Tachypleus tridentatus, a threatened marine arthropod of high ecological and conservation value, is exposed to microplastics in coastal habitats. In this study, fifth-instar and sixth-instar juvenile T. tridentatus were exposed to polystyrene microplastics (PS-MPs) with a diameter of 6 μm at environmentally relevant concentrations (0, 102, 104 particles/L) for 7 and 21 days. Fluorescence imaging revealed the accumulation of PS-MPs within the intestinal tract, with stage - dependent retention patterns across developmental stages. Transcriptomic analysis revealed significant differential gene expression upon PS-MP exposure, with affected pathways related to energy metabolism, immune regulation, stress response, signal transduction, and development. Younger juveniles exhibited stronger transcriptional disturbances in metabolic and stress-related pathways, whereas older juveniles showed more coordinated regulation of homeostatic and adaptive genes. These findings suggest that juvenile T. tridentatus employs instar-dependent molecular response strategies when confronted with microplastic exposure, reflecting developmental differences in stress sensitivity and adaptive capacity. Our study provides new insights into the molecular mechanisms underlying responses to microplastic exposure in an endangered marine species and highlights the importance of incorporating developmental-stage variation into ecological risk assessments and conservation strategies for T. tridentatus in polluted coastal ecosystems.