Spatial patterns of soft coral (Octocorallia) assemblages in the shallow coral reefs of Okinawa Island, Ryukyu Archipelago, Japan: Dominance on highly disturbed reefs

Abstract

Soft corals (Octocorallia) are major and conspicuous benthic components of coral reefs in the Indo-Pacific region. However, despite their major ecological roles, they are still generally understudied. In this study, we investigated the spatial patterns of the community assemblages of soft corals in the waters around subtropical Okinawa Island, Japan. Surveys were conducted at selected sites across an anthropogenic pressure gradient using a hierarchical sampling design to compare soft coral cover and assemblages at different spatial scales. The results revealed significant variations in the mean soft coral cover at different spatial scales, with higher cover in the sites with high levels of anthropogenic pressure. Similarity of percentages analysis showed the overall dominance of Sclerophytum, Lobophytum, and Sarcophyton, genera that are known to have high resiliency to environmental stress. Canonical correspondence analysis also revealed a negative relationship between the dominant soft coral groups and hard coral cover, suggesting that past hard coral mortality events may have set the stage for soft coral dominance. The changes in the relative coverage of benthic components as compared to past studies, specifically the increased dominance of soft corals over hard corals in highly disturbed sites, support this hypothesis. This study provides a detailed look at the spatial patterns of soft coral communities around Okinawa Island and should be useful for projections on the future state of coral reefs as anthropogenic pressures continue to increase.