Effects of short period feeding polysaccharides from marine macroalga, Ulva prolifera on growth and resistance of Litopenaeus vannamei against Vibrio parahaemolyticus infection

Ulva prolifera plays an important part in most green tides around the world. To explore its possible use, the immunostimulatory effect of orally administered U. prolifera polysaccharides in white shrimp, Litopenaeus vannamei, was investigated in this study. Shrimp were divided into four groups: a group fed with the basal diet as the control group (control), and three treatment groups fed respectively with a basal diet supplemented with polysaccharides of cold water extract (PC), polysaccharides of hot water extract (PH), and the filtered residue after the extraction of polysaccharides (R). The highest final weight was found in the group PC (9.31 +/- 0.54g), followed by R (9.11 +/- 0.63g), PH (8.94 +/- 0.61g), and the control group (8.65 +/- 0.53g); the final weight of shrimp in group PC and group R was significantly greater than that in the control group (P<0.05). The minimum feed conversion ratio (FCR) was found in group PC (1.33 +/- 0.02), followed by group R (1.40 +/- 0.04), PH (1.45 +/- 0.04), and the control group (1.54 +/- 0.04), and FCR in PC and PH was significant lower than that in control group (P<0.05). After the feeding trial, the shrimp were challenged with Vibrio parahaemolyticus. Compared to group PH, R, and the control group, the hepatopancreas lysozyme (LSZ) and phenoloxidase (PO) activities of shrimp in group PC were significantly higher from 24 to 96h. Most shrimp died in 24-72h after infection; the shrimp in group PC, PH, R, and the control group at 96h had cumulative mortality of infection of 17.14, 40.00, 34.29, and 54.29%. There were no significances in cumulative mortality between group PH and R (P>0.05); however, they were significantly lower than that in the control group (P<0.05) while they were significantly higher than that in group PC (P<0.05). The highest immune protective rate was found in group PC, followed by group R and PH. There were no significances in immune protective rate between group PH and R; however, they were significantly lower than that in group PC. These results suggest that dietary U. prolifera polysaccharides can promote shrimp growth and enhance non-specific immune and disease resistance enhancement against V. parahaemolyticus in L. vannamei. The findings from this present study will help to promote the application of U. prolifera in aquaculture.