Microplastics disrupt energy metabolism and intestinal integrity in juvenile Chinese horseshoe crabs

As an endangered marine living fossil, the Chinese horseshoe crab ( Tachypleus tridentatus ) inhabits microplastics (MPs) pollution hotspots. This study assessed the toxicity of different concentrations of 5 μm fluorescent polystyrene MPs on two-year-old horseshoe crab through a 14-day exposure and 7-day recovery experiment. The results showed that, (1) MPs accumulated concentration dependently, primarily in gills and intestine. High exposure (200 μg/L) resulted in significant residual burden (0.2109 μg/g), indicating inefficient excretion. (2) Respiratory metabolism was significantly inhibited (decreased oxygen consumption, P < 0.05), impacting energy metabolism and survival. (3) Digestive enzyme activity (lipase, trypsin) was dose-dependently suppressed ( P < 0.05), accompanied by intestinal wall thinning ( P < 0.05) and down-regulation of lipid metabolism genes ( Amy1 , PNLIP ). (4) Lipid metabolism disruption manifested as elevated triglycerides/cholesterol ( P < 0.05), inhibited total lipase ( P < 0.05), and low regulated lipolysis genes ( Mgll-2 , PLD1 ). MPs induced complex toxicity via a “physical damage–metabolic interference–gene regulation” triad. Short-term low-dose exposure triggered compensation, while long-term high-dose exposure caused intestinal damage and metabolic disorder. Incomplete recovery post-exposure to MPs demonstrates cumulative and persistent ecological risks to endangered intertidal species, providing crucial insights for assessing MP impacts on living fossils.