Abstract:The sea cucumber industry has developed rapidly over recent year with breeding areas gradually expanding to the southern Fujian region. Using the warm water of the southern sea area during the winter, Fujian Province has exploited Apostichopus japonicus cage culture. With <1% of the breeding area of the nation, Fujian Province produces 18% of the national sea cucumber output, cementing a new paradigm, known as “north sea cucumber and south cultivation,” which has accelerated sea cucumber industry growth substantially. However, with the rapid development of the sea cucumber breeding industry in this area, breeding diseases have become common in recent years, significantly affecting both the survival and weight gain rates associated with breeding. To understand the causes of diseases in this culture mode and explore prevention and control measures, we investigated a sea cucumber enteritis outbreak in the cage cultures of the Dongwuyang Sea, Fujian Province. We analyzed the epidemiological characteristics, pathogenic pathology, environmental microbial groups, and the structure of the intestinal microbiota of sea cucumbers. The intestinal tract of the diseased sea cucumbers were small and contained a large amount of water. The connective tissue and muscle layer were separated, with the connective tissue cells in the sick intestinal tract swollen and partially destroyed, and loosely arranged. Microbial culture data analysis of cultivable bacteria and intestinal and ambient samples from sea cucumbers were conducted. The total quantity and Vibrio quantity in the intestinal samples of the diseased sea cucumbers were significantly greater than those in the intestinal samples of healthy sea cucumbers. High-throughput sequencing of the 16S rDNA variable region V3–V4 identified 25 phylums and 311 families. Enterobacteriaceae, Bacillaceae, and Lachnospiraceae represented the top three bacteria families in the healthy group, with relative abundances of 8.69%, 5.53%, and 5.12%, respectively, while Rickettsiaceae, Vibrionaceae, and Enterobacteriaceae represented the top three prominent bacterial families in the diseased group, with relative abundances of 12.66%, 8.22%, and 5.71%, respectively. Vibriaceae were more prevalent in the diseased group than in the healthy group. The relative abundance of Vibriococcaceae was 34.99% and a mixed diet led to the highest environmental sample abundance. Principal co-ordinates analysis (PCoA) and unweighted pair-group method with arithmetic mean (UPGMA) cluster analyses indicated that the samples of each group were well grouped, and the microbiota types of the mixed diet were similar to that of the intestinal flora. In conclusion, the intestinal tissue structure of the diseased sea cucumber was changed, and a significant amount of Vibrio bacteria in the intestinal tract primarily derived from the mixed diet, which posed a threat to sea cucumber health. Operational taxonomic units (OTUs) with a relative abundance of 0.5% were mainly located in the Lutimonas, Aliivibrio, and Formosa genera. Vibrio was mostly pathogenic, while Formosa was mostly probiotic. Healthy and diseased gut microbiotas differed significantly in terms of high abundance OTU. These results indicate that the intestinal tissue structure of diseased sea cucumber has changed, and that the large amount of Vibrio in the intestine mainly originates from mixed feed. The large microbial community in sea cucumber feed poses a certain threat to their health. The probiotic content of the intestinal tract of healthy sea cucumbers was higher than that of diseased sea cucumbers, and pathogenic bacterial content in diseased sea cucumbers was higher than that of healthy sea cucumbers. Our study findings provide a scientific basis for the development and utilization of potential probiotics and potential pathogenic bacterial screening. Furthermore, our results support sea cucumber bait optimization and offer a theoretical foundation for scientific breeding and disease management.
