Laminarin is a source of immunostimulants and antioxidative biomolecules involved in supporting the performance and health of aquatic animals. Hence, this study investigated the growth performance, intestinal morphology, blood biomarkers, and immune response of Thinlip Grey Mullet (Liza ramada) fed dietary laminarin. For 60 days, mullets were fed diets supplemented with laminarin at 0, 200, 400, 600, and 800 mg kg−1, then the growth performance was evaluated, and samples were collected. The FBW, WG, SGR, PER, and carcass lipid content were markedly increased, while the FCR was significantly lowered by dietary 600 and 800 mg kg−1. Further, the lipase and protease activities were significantly higher in mullets fed laminarin at 600 mg kg−1 than those fed 0, 200, 400, and 800 mg kg−1. The intestinal histopathological evaluation revealed that all layers of the intestinal villi and the intestinal wall appeared intact without any deteriorating changes. The intestinal mucosal lining of anterior and middle segments showed improved morphological appearance with increased goblet cells in the intestinal villi associated with increased supplemented laminarin level. The total protein, globulin, and total cholesterol were markedly higher in fish fed 400 and 600 mg kg−1 laminarin than those fed 0, 200, and 800 mg kg−1. Furthermore, the lysozyme, catalase, and glutathione peroxidase activities were higher in mullets fed laminarin at 600 mg kg−1 than those fed a laminarin-free diet. The superoxide dismutase was higher in fish fed 200, 400, and 600 mg kg−1than those fed 0 and 800 mg kg−1. On the other hand, the malondialdehyde activity was markedly decreased by 400 and 600 mg kg−1 of dietary laminarin. Overall, dietary laminarin is required at 338–761 mg kg−1 to reveal the best growth performance, intestinal morphology, blood biomarkers, antioxidative, and immune response in mullets.
