Succession of bacterial communities during a disease progress in cuttlefish Sepia pharaonis

The cuttlefish Sepia pharaonis is cultured in large-scale artificial culture due to its high nutritional value. During its culture in 2021 and 2022, when the dorsal mantle length of individual was approximately 2.5 cm, obvious ulcer focus appeared on the back, and the ulcer focus gradually expanded until the cuttlebone fell off and the cuttlefish died. Once one individual showed this symptom, it would be quickly spread to the whole population with a mortality as high as 90%. To address the relationship between the disease outbreak and the microbiota, the bacterial communities in the healthy, diseased, and agonal cuttlefish S. pharaonis were assessed using both bacterial culture and 16S rRNA sequencing technology. Cultivable bacteria showed that an antagonistic phenomenon existed in the healthy cuttlefish S. pharaonis, but the colonies showed a higher diversity in the samples collected from diseased and agonal individuals. The bacterial community was distinctly different among the three groups, characterized by increase in alpha diversity, alteration in bacterial community structures, and the dominant bacteria as the disease progressed. In addition, bacterial genera that can be used as biomarkers, including Photobacterium, Bacillus, and Halarcobacter, are proposed. In conclusion, the balance of the microbiota is necessary for the health of cuttlefish S. pharaonis, and biomarker bacteria might be used in the early detection of disease occurrences.