Phase shift from a stony-coral to a soft-coral community on a coral reef: a case study of an alternative state

ABSTRACT

A phase shift from a stony-coral community to an alternative state represented by octocorals and corallimorpharians was recorded on a coral reef at the Hon Noi Islands (offshore area of south-central Vietnam). For a seven-year period, average stony-coral cover decreased from 80% to 4.7%, diversity of reef-building corals decreased from 169 to 17 species, and in the shallow area of a reef (1–4 m depth), the cover of corallimorpharians (mainly Rhodactis indosinensis) and octocorals (mainly Clavularia sp.) increased to 62%. Mass scleractinian mortality could have resulted from an outbreak of crown-of-thorns starfish, Acanthaster sp., which developed in the surrounding area in the last decade. The only significant contributor to stony-coral recovery on the study reef to date was the blue coral, Heliopora coerulea. Given the serious coral decline on the upstream reefs as potential sources for coral recruitment and competitive advantages of corallimorpharians and octocorals over scleractinians, the prospects for rapid scleractinian recovery of the study site are poor.

KEYWORDS:

• Coral reefs
• alternative state
• corallimorpharians
• octocorals

Acknowledgements

This study was supported by the Joint Vietnam–Russia Tropical Science and Technology Research Center, Ecolan 3.1, Task 4. The authors are grateful to the Sunest Co. for permission to conduct fieldwork off Hon Noi Islands and to the Proof-Reading-Service.com for English language editing of the manuscript. We also thank the two anonymous reviewers for constructive comments on our manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This study was supported by the Coastal Branch of the Joint Vietnam–Russia Tropical Science and Technology Research Center