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Coral reefs worldwide are now under threat of degradation and destruction by ocean acidification and global warming. Local anthropogenic impacts such as eutrophication and turbidity also weaken them. With the degradation of coral reefs, coral reef communities shift from an ecosystem dominated by hard coral to other organisms. Here we consider soft coral as an example of an organism that can replace hard corals in a reef community. In the Ryukyu Islands, soft coral is a typical organism found in coral reefs and is present at a high density. However, there are far fewer published studies of soft coral than of hard corals. Therefore, few studies have quantitatively assessed the potential for a community to shift from hard coral to soft coral. Many studies of hard coral can be applicable to soft coral because they have some points in common, such as their habitat and body structure. We can consider a possible or ongoing community shift in coral reefs by comparing hard and soft coral directly, using the perspectives and techniques of hard coral studies for the study of soft coral. Here we introduce the concept of community shifts in the coral reef community with regard to previous studies. We discuss the possibility and the importance of a community shift from hard to soft coral. The causes and mechanisms of the community shift are considered alongside the biological features and physical structure of soft coral. To investigate the mechanisms involved, we suggest some approaches that can be used to understand the degree of community shift occurring. Finally, we consider how the coral reef ecosystem would be affected by such a community shift. We need to undertake a multidisciplinary approach to understand the changing coral reef habitat. As an example, we suggest how to assess a community shift by collaborative studies of hard and soft coral.

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