Diagnosis of Skeletal Eroding Band (SEB) and spatial observations of coral health and disease in Koh Tao, the Gulf of Thailand

Skeletal Eroding Band (SEB) is a highly virulent coral disease associated with folliculinid ciliates; however, molecular confirmation of lesion-associated organisms remains limited in the Gulf of Thailand. This study combined field-based epidemiological surveys, microscopic examination, and ITS-targeted molecular detection to investigate SEB prevalence and confirm the presence of lesion-associated Halofolliculina spp. on reefs of Koh Tao. Coral health and disease prevalence were assessed across eight reef sites using belt transects, while lesion characterization was observed through microscopic examination. SEB was detected at all survey sites across multiple coral taxa, indicating a broad host range and a widespread occurrence during the survey period. The microscopic observation of SEB lesions revealed dense aggregations of lorica-bearing ciliates that attached to the exposed skeletal surfaces of infected corals. Targeted amplification of the internal transcribed spacer (ITS) region produced 211-bp amplicons exclusively in diseased coral tissues (DD-SEB), with no detection in healthy (HH) or apparently healthy coral tissues (HD-SEB). Sequence comparison showed high similarity to reference Halofolliculina spp. and phylogenetic analysis supported their placement within the Halofolliculina clade. These findings provide molecular support for the presence of Halofolliculina spp. associated with SEB lesions in the Gulf of Thailand and complement visual and microscopic observations. The integration of epidemiological, microscopic, and molecular approaches provides complementary evidence for SEB disease identification and contributes to a better understanding of its ecology in reefs.