PRELIMINARY EVIDENCE FOR DIRECTIONAL ALLELOPATHIC EFFECTS OF THE SOFTCORAL SINULARIA-FLEXIBILIS (ALCYONACEA, OCTOCORALLIA) ON SCLERACTINIAN CORAL RECRUITMENT

Certain species of alcyonacean soft corals have been shown to release toxins which inhibit growth and cause tissue necrosis in selected adul t scleractinian corals in their vicinity. We have now demonstrated exp erimentally that soft corals with such allelopathic capabilities can a ffect recruitment of juvenile scleractinian corals. A large colony of Sinularia flexibilis (Octocorallia, Alcyonacea) was placed in the cent er of each of two steel grids implanted on a shallow reef slope on Orp heus Island, Great Barrier Reef, Australia. Settlement plates were att ached to the grids radiating outward from each soft coral in a uniform pattern. Two other grids with settlement plates arranged similarly, b ut without soft coral colonies, comprised the controls. Scleractinian coral recruitment was assessed against presence or absence of S. flex flexibilis. and as a function of recruitment position with respect to the soft coral colony. Prevailing current direction was measured using a recently developed small current meter, permitting current effects on recruitment to be evaluated. The presence of S. flexibilis was foun d to significantly depress coral recruitment. Settlement plates positi oned down-current from the soft corals exhibited the lowest densities of coral recruitment, and there was a significant negative correlation between recruitment density in each directional quadrant and the prop ortion of time during which current flowed towards that quadrant. Ther e was no correlation between density of juvenile corals and distance f rom the soft coral, probably due to directional variance. Coral spat i ncluded Acropora sp., Pocillopora sp. Seriatopora, Merulina sp., and a Mussidae, but there was no significant difference between the control s and treatments in relation to the taxa found on each grid. The overa ll mortality rates of coral spat were not significantly different betw een treatments and controls. These results show that the soft coral S. flexibilis, known to release allelopathic toxins, can depress recruit ment of scleractinian corals, and that this depression is exercised in a directional manner, depending upon the prevailing currents.