Vertical distribution of the Scleractinian Corals Alveopora japonica (Eguchi, 1965) and Montipora millepora (Crossland, 1995) on Southern Jeju Island, Korea

Abstract

Introduction: Jeju Island, on the southern coast of Korea, has recently experienced increases in high-latitude scleractinian corals, accompanied by shifts in hard-bottom benthic communities. However, the local distribution and condition of the high-latitude corals Montipora millepora and Alveopora japonica at Seopseom remain poorly resolved despite ongoing benthic change. We tested the working hypothesis that their occurrence and condition are structured primarily by depth-related habitat filtering, expressed through variation in light environment, hydrodynamic exposure, and available hard-bottom benthic cover, rather than by demonstrated competitive interaction alone. Methods: Depth-related patterns in epibenthic cover and the occurrence of M. millepora and A. japonica were quantified using underwater photographic transects of 20 m length at 5, 10, and 15 m depth at Seopseom, southern Jeju Island, in 2016 and 2017. Benthic cover categories and coral occurrence, condition, and colony size-frequency patterns were assessed across depths to evaluate habitat-associated distribution patterns. Results: Sargassum sp. dominated the shallow 5 m assemblage and declined with depth, whereas filamentous turf algae and Ecklonia cava increased with depth. Coralline algae, including geniculate and crustose coralline algae, showed non monotonic variation across depths. Montipora millepora occurred at all surveyed depths, whereas A. japonica was recorded only at 10–15 m. Colony size frequency distributions of both species were dominated by small colonies, suggesting recent recruitment at the study site. Discussion: The observed depth-related patterns support the interpretation that habitat filtering, and substrate context are central to structuring the distribution of M. millepora and A. japonica at Seopseom. Competition for hard substrate remains a plausible mechanism, but it was not directly tested in this study and requires explicit contact-frequency, spatial co- occurrence, or interaction-based analyses. These results provide a hypothesis-driven baseline for future monitoring aimed at resolving the ecological and environmental drivers of ongoing benthic community change on Jeju Island.