Bacterial communities associated with the blue dragon nudibranch Glaucilla marginata from Phuket, Thailand

Sampling neustonic nudibranchs is challenging because they live in the open-ocean surface layer, which restricts available microbiome reference data. On July 12, 2025, a mass stranding of the blue dragon nudibranch ( Glaucilla marginata ) at Karon Beach, Phuket, Thailand, provided a rare opportunity for microbiome sampling. We generated a 16S rRNA gene amplicon dataset from whole-body homogenates of stranded individuals. Ten specimens were sequenced as five pooled samples (two individuals per pool) targeting the V3–V4 region on an Illumina MiSeq. After processing, the dataset contained 43 amplicon sequence variants, dominated by a small number of taxa; most samples were enriched in Firmicutes, largely represented by Mycoplasma . This low bacterial richness suggests host-mediated filtering, potentially driven by host-produced antibacterial compounds or competitive exclusion mediated by resident microbes. In addition, six culturable bacterial isolates were obtained and identified by near full-length 16S rRNA gene sequencing. Some isolates were not detected in the amplicon data, underscoring the importance of integrating culture-dependent and -independent approaches to better characterize host-associated assemblages.