Abstract:A shrimp hatchery of Litopenaeus vannamei was infected by an unknown disease during the zoea stage with declined feeding, reduced activity, and increased mortality. In order to diagnose and explore the pathogenic factors causing the zoea disease, we collected samples from two diseased rearing ponds at zoea stage 3 (Z3) and zoea stage 1 (Z1), respectively. PCR detection for nine known pathogens, including WSSV (White spot syndrome virus), IHHNV (Infectious hypodermal and hematopoietic necrosis virus), VpAHPND (Acute hepatopancreatic necrosis disease caused by Vibrio parahaemolyticus), TSV (Taura syndrome virus), IMNV (Infectious myonecrosis virus), YHV (Yellow head virus), EHP (Microsporidian Enterocytozoon hepatopenaei), CMNV (Covert mortality nodavirus), and SHIV (Shrimp hemocyte iridescent virus) showed negative results. Histopathological diagnosis showed unknown brown particles in the epithelial cells of the hepatopancreatic tubules. The results of bacterial isolation and identification for potential pathogens resulted in only two isolates of Vibrio alginolyticus on 2216E media. The average survival rate of gnotobiotic brine shrimp (Artemia franciscana) nauplii immersion- challenged with these isolates were 58% and 83% respectively. The microbial communities were profiled by high-throughput sequencing of the V3~V4 hypervariable region of 16S rRNA gene using the Illumina HiSeq sequencing platform. Meta-16S-sequencing revealed the microbial diversities and their relative abundance in the zoea stage of L. vannamei. The results showed that Proteobacteria, Bacteroidetes, and Firmicutes were the most dominant phyla in the entire bacterial community from both samples, but their relative abundance was obviously different. At the genus level, Vibrio was the most abundant genus in both samples, at 74.3% and 60.5%, respectively, and the relative abundance of Tenacibaculum (21.9%) was the second highest genus in the Z1 stage sample. Collectively, the information regarding the management of larval rearing and the above results of the case study imply that the disease may be caused by an opportunistic infection with some strains of V. alginolyticus having moderate pathogenicity under the dystrophia of larva caused by quality degradation of larval feed due to the expiration of storage.
