Morphological characterization and multilocus phylogenetic placement of Physalia utriculus (Siphonophore: Physaliidae) from the Gulf of Thailand

Physalia is a group of pleustonic siphonophores renowned for wind-current driven dispersal, and is a recurring public-health and tourism concern on Thai beaches. Addressing the problem has been hindered by the unresolved taxonomy and distribution of Physalia in Thai waters for decades. This study provides the specimen-based multilocus characterization of Physalia utriculus from the Gulf of Thailand, complementing earlier morphology-based Thai records, by integrating morphological characterization with multi-gene (16S, 18S, and COI) analysis and a COI haplotype network. Specimens collected from Chalathat and Samila beaches of the Songkhla province, exhibited the following diagnostic features: an asymmetrical blue-green triangular pneumatophore with a rugose crest (15–38 wrinkles), one principal tentacle, and flask-shaped gastrozooids. Phylogenetic reconstruction placed the Thai specimens within the global clade of P. utriculus, supporting their assignment to this species on the basis of phylogenetic affinity. The median-joining COI haplotype network of 85 Indo–Pacific and Atlantic individuals resolved 56 haplotypes. Thai specimens shared the two most common, geographically widespread haplotypes with other Indo–Pacific populations but also carried a high proportion (11 of 13) of locally restricted haplotypes. Analysis of molecular variance revealed no significant COI differentiation was detected among the four well-sampled Indo-Pacific regions, although this inference is constrained by uneven geographic sampling and the inclusion of only one Atlantic sequence. Monthly mean wind and surface-current fields indicate that the February 2026 stranding occurred under northeast-monsoon conditions, but these climatological summaries do not identify a source region or reconstruct individual transport trajectories. Together, the morphological and multilocus data provide a specimen-based baseline for P. utriculus in the Gulf of Thailand.