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.