A Sea Anemone Outbreak Eliminates Damselfish Territories from Fringing Reefs in Southern Taiwan

In 2000, when the sea anemone, Mesactinia ganesis, was rare on fringing reefs in an embayment in southern Taiwan (21'57 degree N; 120'45 degree E) (Jan et al. 2007), bluntsnout gregory Stegastes lividus (Bloch and Schneider) (Fig. 1a) and dusky gregory, S. nigricans (Lacepede) (Jan et al. 2003) had long-established territories on coral branches on the reef flat. An outbreak of the sea anemone has occurred in recent years in this embayment. Colonies of branching corals on the slanting reef surface were eliminated by the spreading M. ganesis. Strong territoriality of the damselfishes has slowed down the elimination process, as remaining coral colonies can still be found within their territories (Fig. 1b). Nevertheless, currently more than 80% of the former damselfish territories have already disappeared from this part of the reef (Fig. 2). Some holders emigrated to the lower part of a near-by reef, competing for new territories on the few subsisting coral colonies (Fig. 1c). In parallel, relic coral skeletons from abandoned damselfish territories have been taken over by Dick's damsel, Plectroglyphidodon dickii (Lienard), as nesting substrate. This observation indicates that the sea anemone outbreak has resulted in drastic changes in the demography of habitat specialist in fishes. While the outbreak has shown no signs of recession, the fish community, along with other fouling organisms, will likely remain unstable in this embayment.