EFFECTS OF SOFT CORALS ON SCLERACTINIAN CORAL RECRUITMENT .1. DIRECTIONAL ALLELOPATHY AND INHIBITION OF SETTLEMENT

Experiments were performed on the Great Barrier Reef, at Orpheus Islan d (18 degrees 40' S, 145 degrees 30' E) and Lizard Island (14 degrees 41' S, 145 degrees 28' E), Australia, to examine allelopathic effects of soft corals on the larval recruitment of scleractinian corals. Cera mic tiles were used as settlement plates and arranged around the soft corals Sinularia flexibilis (Quoy and Gaimard) and Sarcophyton glaucum (Quoy and Gaimard), serving as treatments. One control consisted of s tacks of settlement plates uninfluenced by any organism, while a secon d control had settlement stacks surrounding a scleractinian coral to c ontrol for depletion of larvae via feeding. Coral spat recruitment was approximately 7 times higher at Lizard Island (7032) than at Orpheus Island (1038). The pattern of coral recruitment and relative abundance s of coral recruits around the controls and soft corals, however, was similar at the 2 sites. The average density of coral spat was always s ignificantly less around the soft corals than the controls, indicating that soft corals at each experimental site inhibited scleractinian co ral 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 respec t to the distance from the soft coral and the direction of the prevail ing 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 co ral spat were exposed to settlement plates impregnated with the extrac t of S, flexibilis and control plates. Settlement only occurred on the untreated controls.