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Not AvailableA130-day culture experiment was performed to compare the effects of rearing Litopenaeus vannamei in a biofloc system and conventional autotrophic system, supplemented with diets having graded protein levels on growth performance, non-specific immune response and immunomodulatory activity. The experiment groups consisted of four protein-level treatments with four replicates each, and was performed in 16 experimental units (7.5- Tcapacity) stocked with juvenile L. vannamei (initial average weight 1.48 ± 0.4 g) at a density of 150 juvenile shrimps /m3. Biofloc-based groups fed with supplementary feed containing varying levels of crude protein (CP) BFT40 (40% CP), BFT32 (32% CP) and BFT24 (24% CP), and a control group (CP 40%), were maintained in an autotrophic system. At the end of the experimental period, there was an improvement of 32.6–52.6% in terms of productivity, 22–27.6% average body weight; 8.7–19.6% survival rate and 10–31% feed conversion ratio (FCR) in shrimp maintained in biofloc systems as compared to control. Similarly, the protein efficiency ratio (PER) in biofloc treatment groups was32 to 83% higher than control. The shrimp also showed higher resistance to disease in the biofloc treatment when challenged with pathogenic Vibrio parahaemolyticus, as survival of juveniles was significantly higher (p < 0.05) in the biofloc treatments than in the control group. The total hemocyte count and proPhenoloxidase activity were significantly increased in the biofloc treatments when compared to control. The mRNA profiling of important immune genes in the shrimps showed higher levels of expression in the 32 and 40% CP feed treatments. Superoxide dismutase (SOD) mRNA transcript levels were highly up-regulated in the BFT40 treatment (7-fold), moderately upregulated in the BFT32 treatment (2-fold), and downregulated in the BFT24 treatment (0.9-fold). Transcription of antimicrobial peptides like crustin showed significant up-regulation in BFT40 (10.31-fold) followed by BFT32 (6.11-fold) and BFT24 (1.07-fold). Likewise, other immune genes, such as MnSOD, hemocyanin, proPhenoloxidase (proPO), peroxinectin (PX) and serine protease (SP) showed similar trends, indicating immunomodulation in the biofloc groups. We concluded that the biofloc system has the potential to improve growth performance, immune response and disease resistance in Pacific white shrimp in spite of the low-protein supplemented dietNot Availabl