Effects of Alexandrium pacificum Exposure on Exopalaemon carinicauda: Hepatopancreas Histology, Antioxidant Enzyme Activity, and Transcriptome Analysis

<i>Alexandrium pacificum</i>, a dinoflagellate known for causing harmful algal blooms (HABs), has garnered significant attention due to its potential toxicity to marine ecosystems, fisheries, and human health. However, the effects of this toxin-producing alga on shrimp are not yet comprehensively understood. This study aimed to assess the hepatopancreas damage induced by <i>A. pacificum</i> in the economically important shrimp species <i>E. carinicauda</i> and to elucidate the underlying molecular mechanisms through histology, antioxidant enzyme activity, and transcriptome analysis. The shrimp were assigned to either a control group or an exposed group, with the latter involving exposure to <i>A. pacificum</i> at a concentration of 1.0 × 10⁴ cells/mL for 7 days. A histological analysis subsequently revealed pathological changes in the hepatopancreas tissue of the exposed group, including lumen expansion and the separation of the basement membrane from epithelial cells, while antioxidant enzyme activity assays demonstrated that exposure to <i>A. pacificum</i> weakened the antioxidant defense system, as evidenced by the reduced activities of catalase, superoxide dismutase, and glutathione, along with increased malondialdehyde levels. Transcriptome analysis further identified 663 significantly upregulated genes and 1735 significantly downregulated ones in the exposed group, with these differentially expressed genes being primarily associated with pathways such as protein processing in the endoplasmic reticulum, mitophagy, glycolysis/gluconeogenesis, sphingolipid metabolism, and glycerophospholipid metabolism. This study provides novel insights into the toxicological effects of <i>A. pacificum</i> on aquatic organisms and enhances the current understanding of the ecotoxicological risks posed by HABs.