The immunostimulatory effects of hot-water extract of Gelidium amansii via immersion, injection and dietary administrations on white shrimp Litopenaeus vannamei and its resistance against Vibrio alginolyticus

The total haemocyte count (THC), phenoloxidase activity, and respiratory burst were examined when white shrimp Litopenaeus vannamei were immersed in seawater (34‰) containing hot-water extract of red alga Gelidium amansii at 200, 400 and 600 mg l−1, injected with hot-water extract at 4 and 6 μg g−1 shrimp, and fed diets containing hot-water extract at 0, 0.5, 1.0 and 2.0 g kg−1. These parameters increased significantly when shrimp were immersed in seawater containing hot-water extract at 400 and 600 mg l−1 after 1 h, when shrimp were injected with hot-water extract at 6 μg g−1 shrimp after one day, and when shrimp were fed diets containing hot-water extract at 1.0 and 2.0 g kg−1 after 14 days. Phagocytic activity and clearance efficiency were significantly higher for the shrimp that were fed diets containing hot-water extract at 1.0 and 2.0 g kg−1 than those of shrimp that were fed diets containing hot-water extract at 0 and 0.5 g kg−1 after 14 and 28 days. In a separate experiment, L. vannamei which had received hot-water extract via injection, or fed diets containing hot-water extract, were challenged after 3 h or 28 days with V. alginolyticus at 2 × 106 cfu shrimp−1 and 1 × 106 cfu shrimp−1, respectively, and then placed in seawater. The survival of shrimp that were injected with hot-water extract at 6 μg g−1 was significantly higher than that of control shrimp after 1 day, and the survival of shrimp fed diets containing hot-water extract at 0.5, 1.0 and 2.0 g kg−1 increased significantly after 3 days as well as at the end of the experiment (6 days after the challenge), respectively. It was concluded that L. vannamei that were immersed in hot-water extract at 400 mg l−1, injected with hot-water extract at 6 μg g−1 shrimp, and fed hot-water extract of G. amansii at 2.0 g kg−1 or less showed increased immune ability as well as resistance to V. alginolyticus infection.