Dietary Ferrous Sulfate Enhances Resistance to Vibrio splendidus-Induced Skin Ulceration in Apostichopus japonicus via Immune and Antioxidant Modulation.

The sea cucumber (Apostichopus japonicus) is a commercially important marine species. However, its survival is increasingly threatened by frequent outbreaks of Skin Ulceration Syndrome caused by Vibrio splendidus. This study evaluated the effects of dietary supplementation with ferrous sulfate (FeSO₄) at two concentrations (0.5% and 1%) over short-term (21 days) and long-term (56 days) feeding periods on immune defense, antioxidant capacity, and resistance to V. splendidus infection. Key parameters measured included survival rate, cellular immune activity, antioxidant enzyme levels, and expression of immune-related genes. Long-term (56 days) supplementation with 1% FeSO₄ significantly improved survival after infection (90 ± 4.7%). Phagocytic activity and respiratory burst were enhanced by approximately ~1.9-fold and ~1.8-fold, respectively (p < 0.05). The expression of sod, ferritin, and hsp70 genes was upregulated by ~2.1-fold, ~2.0-fold, and ~1.6-fold, respectively (p < 0.05). These results indicate strengthened cellular immunity and antioxidant capacity. Long-term (56 days) supplementation with 0.5% FeSO₄ increased lysozyme activity (~1.3-fold) and c3 expression (~4-fold) (p < 0.05), thereby enhancing humoral immunity. In contrast, short-term (21 days) supplementation increased ACP and AKP activities by approximately ~2-fold each, and LZM activity by ~1.2-fold (p < 0.05). However, it did not significantly improve survival, indicating limited protective effects. Overall, 56-day dietary supplementation with FeSO₄, particularly at 1%, effectively enhances immune and antioxidant responses in A. japonicus. This supplementation represents a promising strategy for preventing V. splendidus-induced skin ulceration in aquaculture.