Effects of pristine and UV-aged polystyrene nanoplastics on post-molt horseshoe crabs Tachypleus tridentatus: Aging-mediated divergence in biological effects.

The Chinese horseshoe crab (Tachypleus tridentatus), an intertidal chelicerate with a 400-million-year evolutionary history, is increasingly exposed to anthropogenic pollutants in its coastal nursery habitats. Because of the ecological importance of the species and the vulnerability of the post-molt stage, we investigated the effects of pristine and 7-day UV-aged polystyrene nanoplastics (NPS) on juvenile T. tridentatus at an environmentally relevant nominal concentration of 100 μg/L. Fourier transform infrared spectroscopy confirmed surface oxidation of aged NPS, as reflected by significantly elevated carbonyl (C=O), hydroxyl (O-H), and carbon-oxygen (C-O) indices. Dynamic light scattering in 25 ppt artificial seawater further showed that aged NPS had a larger hydrodynamic diameter (111.90 ± 3.41 vs. 102.27 ± 1.01 nm) and a more negative zeta potential (-10.32 ± 0.86 vs. -5.65 ± 0.66 mV) than pristine NPS. Both NPS treatments elevated MDA levels, whereas most other biomarkers showed no significant changes. Notably, only aged NPS significantly increased total antioxidant capacity, suggesting a compensatory antioxidant response; nevertheless, pristine NPS elicited stronger overall biomarker perturbation, as reflected by its higher cumulative directional probability (85.4% vs. 71.0%) and Integrated Biomarker Response index (pristine: 3.645 ± 0.645; aged: 1.799 ± 0.679; control: 0.889 ± 0.635). Collectively, these findings demonstrate that UV aging alters both the physicochemical properties and the biological effects of polystyrene nanoplastics, with pristine NPS inducing greater biomarker perturbation in post-molt juvenile T. tridentatus.