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.