Juvenile tri-spine horseshoe crabs (Tachypleus tridentatus) were exposed to determine the effects of single and combined stresses of polystyrene nanoplastics (nano-PS) and heavy metal (Cu2+) on antioxidant enzyme parameters. The juveniles were exposed to a 21-day 100-nm polystyrene concentration (104 particles l-1) and a concentration of Cu2+ (10 µg l-1) followed by a recovery period of 7 days. The in vivo antioxidant activity for whole horseshoe crab was analyzed. The results revealed that all antioxidant parameters, i.e., superoxide dismutase (SOD), catalase (CAT), glutathione (GSH), malondialdehyde (MDA), and lipid peroxidation (LPO), showed both increased and decreased levels in different experimental groups of horseshoe crabs having different experimental conditions compared to the control group at three time points, i.e., on days 7, 14, and 21. Similarly during the recovery period, SOD, CAT, and MDA showed decreased levels in all experimental groups, while GSH and LPO showed increased levels in all experimental groups of horseshoe crabs under the influence of different experimental conditions of nanoplastics and heavy metals compared to the control group on day 28. These results showed that the exposure of nano-PS and Cu2+ had precise effects on juvenile horseshoe crabs. Integrated biomarker responses showed that nano-PS and Cu2+ had adverse effects on juvenile horseshoe crabs. By principal component analysis, the potentially toxic effects of nano-PS and Cu2+ on horseshoe crabs were obtained.

Anthropogenic elevation of atmospheric carbon dioxide (CO2) drives global-scale ocean acidification (OA), which has aroused widespread concern for marine ecosystem health. The tri-spine horseshoe crab (HSC) Tachypleus tridentatus has been facing the threat of population depletion for decades, and the effects of OA on the physiology and microbiology of its early life stage are unclear. In this study, the 1st instar HSC larvae were exposed to acidified seawater (pH 7.3, pH 8.1 as control) for 28 days to determine the effects of OA on their growth, molting, oxidative stress, and gut microbiota. Results showed that there were no significant differences in growth index and molting rate between OA group and control group, but the chitinase activity, β-NAGase activity, and ecdysone content in OA group were significantly lower than those of the control group. Compared to the control group, reactive oxygen species (ROS) and malondialdehyde (MDA) contents in OA group were significantly increased at the end of the experiment. Superoxide dismutase (SOD), catalase (CAT), and alkaline phosphatase (AKP) activities increased first and then decreased, glutathione peroxidase (GPX) decreased first and then increased, and GST activity changed little during the experiment. According to the result of 16S rRNA sequencing of gut microbiota, microbial-mediated functions predicted by PICRUSt showed that “Hematopoietic cell lineage,” “Endocytosis,” “Staphylococcus aureus infection,” and “Shigellosis” pathways significantly increased in OA group. The above results indicate that OA had no significant effect on growth index and molting rate but interfered with the activity of chitinolytic enzymes and ecdysone expression of juvenile horseshoe crabs, and caused oxidative stress. In addition, OA had adverse effects on the immune defense function and intestinal health. The present study reveals the potential threat of OA to T. tridentatus population and lays a foundation for the further study of the physiological adaptation mechanism of juvenile horseshoe crabs to environmental change.

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