Carsten Tsun Ka Kwok ¹ Ray Chun-Wai Yu ¹ Pak-Ting Hau ¹ Karry Yuen Ching Cheung ¹ Iain Chi Fung Ng ¹ Joshua Fung ¹ Ivan Tak Fai Wong ¹ Miranda Yau ² Wai Man Liu ³ Hang-Kin Kong ¹ Kit Hang Gilman Siu ¹ Franklin Wang-Ngai CHOW ¹ Sai Wang Seto ^1*

• ¹ Hong Kong Polytechnic University, Kowloon, Hong Kong, SAR China
• ² Pamela Youde Nethersole Eastern Hospital, Hong Kong, China
• ³ AI Farming (Hong Kong) Limited, Hong Kong, China

Vibrio alginolyticus is a gram-negative, rod-shaped bacterium belonging to the family of Vibrionaceae, a common pathogen in aquaculture animals, However, studies on its impact on Scylla serrata (mud crabs) are limited. In this study, we isolated V. alginolyticus SWS from dead mud crab during a disease outbreak at a Hong Kong aquaculture farm, which caused up to 70% mortality during summer. Experimental infection, and histopathology were used to investigate the pathogenicity of V. alginolyticus SWS in S. serrata and validate Koch's postulates.Comprehensive whole genome analysis and phylogenetic analysis antimicrobial susceptibilities testing, and biochemical characterization were also performed. Our findings showed that V. alginolyticus SWS caused high mortality (75%) in S. serrata with infected individuals exhibiting inactivity, loss of appetite, decoloured and darkened hepatopancreas, gills and opaque muscle in the claw. Histopathological analysis revealed tissue damage and degeneration in the hepatopancreas, gills and claw muscle, suggesting direct and indirect impacts of V. alginolyticus SWS infection. This study provides a comprehensive characterization of V. alginolyticus SWS as an emerging pathogen in S. serrata aquaculture. Our findings underscore the importance of ongoing surveillance, early detection, and the development of targeted disease management strategies to mitigate the economic impact of vibriosis outbreaks in mud crab aquaculture.