Prolonged exposure to hypoxia inhibits the growth of Pacific abalone by modulating innate immunity and oxidative status

In aquatic animals, hypoxia is associated with growth retardation, impaired immunity, susceptibility to pathogens, oxidative stress, and mortality. However, the relative long-term effects of hypoxia on bivalves, including abalone, are not well understood. In this study, we examined the effects of exposure to hypoxic (2.5 and 4 mg O₂ L^-1) and normoxic (8 mg O₂ L^-1) conditions on the growth, survival, and immune and antioxidant responses of the economically important Pacific abalone Haliotis discus hannai over a 4 month period. We observed that exposure to 2.5 mg O₂ L^-1 resulted in marked reductions in assessed shell parameters, average meat weight, and survival compared with exposure to 4 and 8 mg O₂ L^-1. There were also significant reductions in oxygen consumption and ammonia-N excretion in abalone exposed to 2.5 mg O₂ L^-1. We also detected initial immunosuppression in the 2.5 mg O₂ L^-1-treated abalone, as evidenced by a significant reduction in total hemocytes and inhibition of antibacterial and lysozyme activities. Furthermore, intracellular malondialdehyde concentrations were significantly elevated at 1 month in the 2.5 mg O₂ L^-1 treatment group, whereas there were reductions in the levels of glutathione and enzymatic activities of catalase and superoxide dismutase, thereby indicating potential hypoxia-induced oxidative stress and a depression of antioxidant capacity. After 4 months of treatment, severe hypoxia (2.5 mg O₂ L^-1) had significantly modulated all measured parameters, whereas exposure to 4 and 8 mg O₂ L^-1 had induced no significant effects. Collectively, our observations indicate that under long-term exposure to hypoxia, Pacific abalone failed to maintain an effective antioxidant defense system and adequate immunity, with the observed biochemical disruptions leading to a reduction in growth and survival.

Keywords: Abalone; Growth; Hypoxia; Immunity; Oxidative stress; Toxicity.