Effects of soft corals on scleractinian coral recruitment. I:Directional allelopathy and inhibition of settlement

Experiments were performed on the Great Barrier Reef, at Orpheus lsland (18" 40'S, 145" 30' E) and Lizard Island (14" 41' S, 145" 28' E), Australia, to examine allelopathic effects of soft corals on the larval recruitment of scleractinian corals.Ceramic tiles were used as settlement plates and arranged around the soft corals Sinularia flexibilis (Quoy & Gaimard) and Sarcophyton glaucurn (Quoy & Gaimard), serving as treatments.One control consisted of stacks of settlement plates uninfluenced by any organism, while a second control had settlement stacks surrounding a scleractinian coral to control for depletion of larvae via feeding.Coral spat recruitment was approximately 7 times higher at Lizard Island (7032) than at Orpheus lsland (1038).The pattern of coral recruitment and relative abundances of coral recruits around the controls and soft corals, however, was similar at the 2 sites.The average density of coral spat was always significantly less around the soft corals than the controls, indicating that soft corals at each experimental site inhibited scleractinian coral recruitment.Coral recruitment levels were very similar for the 2 control treatments.Coral spat were asymmetrically distributed around the soft corals, and the distribution varied significantly with respect to the distance from the soft coral and the direction of the prevailing current at both Orpheus and Lizard Island.There was a significant negative correlation between coral spat density and current direction under both soft coral treatments and at both sites.No such response was observed in the controls.In a second experiment, scleractinian coral spat were exposed to settlement plates impregnated with the extract of S. flexibilis and control plates.Settlement only occurred on the untreated controls.