Abstract: Octocorals belong to the phylum Cnidaria and the class Octocorallia. Widely distributed across the world’s oceans, they are important components of coral reef ecosystems. However, systematic studies of octocorals in the Xisha Islands and Hainan Island regions have long been lacking. Since 2018, our research team has been conducting surveys of octocorals in the waters surrounding Hainan Island and participated in three coral reef expeditions to the Xisha Islands in 2020, 2023, and 2024. Based on comprehensive analysis of collected specimens, using both morphological and molecular evidence, we identified 111 octocoral species from the Xisha Islands, belonging to 21 families and 44 genera. Of these, the vast majority—100 species from 18 families and 39 genera—are newly recorded for the Xisha Islands, while 15 species from 4 genera and 1 family represent new records for China. In the waters around Hainan Island, we identified 39 species from 7 families and 10 genera, about half of which—20 species from 2 families and 6 genera—are new records for Hainan Island. Depth distribution analysis of octocorals in the Xisha Islands reveals that 41 species occur exclusively in shallow waters (<30 m), and most of these (29/41, 71%) harbor symbiotic zooxanthellae. Another 19 species are found in both shallow and mesophotic zones, with 74% (14/19) also containing zooxanthellae. In contrast, 51 species occur only in the mesophotic zone (30-110 m), the majority of which (41/51, 80%) lack symbiotic algae. Notably, of the 15 species newly recorded for China from the Xisha Islands, 12 species (80%) are restricted to the mesophotic zone, highlighting the historical lack of octocoral surveys at these depths. In addition, considering recent substantial revisions to the taxonomy of octocorals, this study compiles an updated checklist of octocoral species in China based on the latest classification system. The checklist includes 2 orders, 39 families, 88 genera, and 496 species, of which 135 species (27%) have had their original scientific names revised or are no longer accepted.

Key words: Octocorals, Xisha Islands, Hainan Island, Mesophotic zone, Species checklist