Administration of hot-water extract of brown seaweed Sargassum duplicatum via immersion and injection enhances the immune resistance of white shrimp Litopenaeus vannamei

The total haemocyte count (THC), phenoloxidase activity, and respiratory burst were examined when the white shrimp Litopenaeus vannamei (10.42±1.39 g) were immersed in seawater (34‰) containing hot-water extract of brown alga Sargassum duplicatum at 100, 300 and 500 mg l−1, or injected with hot-water extract of S. duplicatum at 2, 6, 10 and 20 μg g−1. These parameters increased significantly when the shrimp were immersed in seawater containing hot-water extract at 300 and 500 mg l−1 after 1 h, or when the shrimp were injected with hot-water extract at 10 and 20 μg g−1 after 1 day. L. vannamei that were injected with hot-water extract at 6, 10 and 20 μg g−1 had increased phagocytic activity and clearance efficiency to V. alginolyticus after 1–6 days. In another experiment, L. vannamei which had been immersed in seawater containing hot-water extract at 100, 300 and 500 mg l−1, or injected with hot-water extract at 2, 6, 10 or 20 μg g−1 were challenged with V. alginolyticus at 1×106, or 1.4×106 colony-forming units (cfu) shrimp−1, and then placed in seawater. The survival of shrimp that received hot-water extract at either dose was significantly higher than that of control shrimp after 2 days, as well as at the termination of the experiment (6 days after the challenge). It is therefore concluded that L. vannamei that were immersed in hot-water extract of S. duplicatum at 300 mg l−1, or the shrimp that were injected with hot-water extract at 10 μg g−1 or less had increased immune ability as well as resistance to V. alginolyticus infection.