As an important aquaculture bivalve, the Pacific oyster (Crassostrea gigas) undergoes a metamorphosis process during its embryonic development. The mortality rate of the eye-spot larva during the metamorphosis and settlement stage is relatively high, which urgently needs to be solved. Glutathione (GSH), as a key antioxidant and immune regulator in organisms, plays an important role in the growth, development, and immune defense of marine invertebrates. Analysis of oyster transcriptome data revealed that the expression of key enzyme-producing GSH, Glutathione S-transferase Kappa 1, reached the highest level at the pediveliger larvae (the stage just before eye-spot larvae), suggesting its important role in the larval settlement process. This study investigated the effects of reduced glutathione supplementation in pediveliger larvae on the settlement metamorphosis and immune capacity of eye-spot larvae. The results showed that high concentrations of glutathione (100 μmol/L, 500 μmol/L) significantly reduced the larval eye-spot rate and settlement rate, with larvae in the 500 μmol/L group nearly all dead after 48 h. Meanwhile, low-concentration GSH (7 μmol/L) could significantly increase the settlement rate of larvae, which was 1.17 times (p < 0.05) that of the control group. When the larvae developed to 1-month-old eye-spot larvae, the survival rate of larvae in the low GSH group significantly increased (38.42 ± 15.98%) after Vibrio splendidus stimulation, while they were almost all dead in the control group. Low GSH could also significantly increase lysozyme LZM activity (by 1.39 times, p < 0.05) and Cu/Zn-SOD activity (by 1.15 times, p < 0.05). Gene expression analysis found that low GSH promoted expressions of larval settlement and metamorphosis-related genes, including ecdysone receptor, P450, and calmodulin. Treatment with low GSH also enhanced the expressions of immune genes of SOD, CAT, and LZM. In summary, a low glutathione supplement can promote the settlement process of oyster eye-spot larvae and enhance disease resistance. These findings reveal the role of glutathione in the metamorphosis and settlement of oyster larvae, providing a scientific basis for optimizing techniques for oyster seedling cultivation.
Li et al. (Tue,) studied this question.