Competitive strategies of soft corals (Coelenterata: Octocorallia): Allelopathic effects on selected scleractinian corals

Abstract

A striking retardation of grwoth was observed in the scleractinian coralPavona cactus (Coelenterata: Scleractinia) growing in the vicinity of the soft coralSinularia flexibilis (Coelenterata: Alcyonacea). More extensive field observations of naturally occurring interactions between soft corals and scleractinian corals suggested that members of the former group can be the more effective competitors for space on hard substrate. To test this hypothesis, colonies of three soft corals,Lobophytum pauciflorum, Sinularia pavida, andXenia sp. aff.danae, were relocated next to stands of two hard corals,Pavona cactus andPorites andrewsi (=Porites cylindrica), and compared with undisturbed control areas. In areas where soft corals and scleractinian corals were in direct contact, significantly high levels of local mortality in the latter occurred in three of the six interaction pairs. One soft coral,L. pauciflorum, also caused extensive and significant mortality inPorites andrewsi in a non-contact situation. The scleractinian corals had no effect on the soft corals considered here. These results indicate that soft corals can effectively compete for space against hard corals. Furthermore, it is inferred that toxic exudates from the soft coral might be responsible for causing localized mortality in hard corals, since extensive mortality occurred in certain cases in the absence of contact. Competitive abilities of soft corals in interactions with hard corals varied in a species-specific manner. Susceptibility of hard corals to competitive mechanisms utilized by soft corals, particularly allelopathic ones, likewise varied species-specifically. It is commonly believed that the adaptive value of toxic compounds in soft corals stems from their effectiveness as a chemical defence mechanism in predator-prey interactions. This study has demonstrated their further role as allelopathic agents in interspecific competitive interactions.