Growth, immunity and transcriptome response to different stocking densities in Litopenaeus vannamei

The effects of different stocking densities on Litopenaeus vannamei were investigated from the aspects of growth performance, immune response and transcriptome in this experiment. L. vannamei (initial body weight: 0.30 ± 0.02 g) were reared for 8 weeks at three stocking densities of 100 (LSD), 200 (MSD) and 300 (HSD) shrimp/m³, respectively. The results showed that the survival rate (SR), final body weight (FBW), weight gain rate (WGR), specific growth ratio (SGR) and protein efficiency ratio (PER) of L. vannamei significantly decreased, while the feed factor (FCR) significantly increased with the increase of stocking density. After Vibrio parahemolyticus infection, the SR of L. vannamei in the HSD group was significantly lower than that in the LSD and MSD groups. Increasing stocking density significantly increased the activities of aspartate aminotransferase (AST), alanine aminotransferase (ALT) and lysozyme (LYS) while significantly decreased the activities of catalase (CAT) and phenol oxidase (PO) in the serum of L. vannamei. Similar changes of the gene expression as the activities of immune enzymes were found in the hemocytes. Pairwise comparison between the LSD, MSD and HSD group in the transcriptome analysis identified that there were 304, 1376 and 2083 differentially expressed genes (DEGs) in LSD vs MSD, MSD vs HSD and LSD vs HSD, respectively. Among them, most of the immune-related DEGs were down-regulated and metabolism-related DEGs were up-regulated with the increasing stocking density. In addition, KEGG enrichment pathway analysis revealed that several immune and metabolic related pathways including PI3K-Akt signaling pathway and AMPK signaling pathway were significantly enriched. Of these, the PI3K-Akt signaling pathway had the most DEGs and was also the most significantly enriched pathway. Furthermore, 16 DEGs (such as FOXO, PCK2 and CTSC, etc.) and partial immune enzyme activity (such as AST, CAT and PO, etc.) changes were closely correlated with the increase of stocking density when partial immune-related DEGs and immune-related enzymes were analyzed jointly. All these results indicated that changes in stocking density had a significant effect on the growth performance, immunity and transcriptome of L. vannamei.