Phylogenetic Characterization of Vibrio harveyi and Photobacterium damselae Isolated from Cultured Grouper in Hainan, China, During 2024–2025

Grouper is the primary mariculture fish species in China, yet it is currently plagued by severe bacterial diseases caused by Vibrio harveyi and Photobacterium damselae. To characterize the phylogenetic relationships of V. harveyi- and P. damselae-derived cultured grouper on Hainan Island, Twenty-eight isolates (17 V. harveyi and 11 P. damselae) were recovered from moribund leopard coral grouper (Plectropomus leopardus) and hybrid grouper (Epinephelus fuscoguttatus × E. lanceolatus) sampled across farming areas during 2024–2025. Isolates were identified by 16S rRNA gene sequencing, compared with GenBank reference strains, and analyzed using neighbor-joining phylogenies. V. harveyi isolates occurred on multiple branches and were interspersed with reference strains from other Chinese regions. In contrast, 10 of the 11 P. damselae isolates from Hainan grouped within a major clade. Among the sampled isolates, neither species showed an immediately apparent spatial or seasonal pattern in its distribution across the 16S rRNA gene-based phylogenetic trees. However, the limited sampling and discriminatory power of the 16S rRNA gene do not support definitive conclusions regarding geographic or seasonal structuring. These findings provide preliminary information on the phylogenetic relationships of bacterial isolates associated with cultured groupers in Hainan and may inform future pathogen surveillance. Further studies using broader sampling and higher-resolution molecular approaches are needed to clarify the genetic relationships among these isolates.