Preliminary Evidence for Directional Allelopathic Effects of the Soft Coral 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 adult scleractinian corals in their vicinity. We have now demonstrated experimentally that soft corals with such allelopathic capabilities can affect recruitment of juvenile scleractinian corals. A large colony of Sinularia flexibilis (Octocorallia, Alcyonacea) was placed in the center of each of two steel grids implanted on a shallow reef slope on Orpheus Island, Great Barrier Reef, Australia. Settlement plates were attached to the grids radiating outward from each soft coral in a uniform pattern. Two other grids with settlement plates arranged similarly, but without soft coral colonies, comprised the controls. Scleractinian coral recruitment was assessed against presence or absence of S. 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 found to significantly depress coral recruitment. Settlement plates positioned 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 proportion of time during which current flowed towards that quadrant. There was no correlation between density of juvenile corals and distance from the soft coral, probably due to directional variance. Coral spat included Acropora sp., Pocillopora sp. Seriatopora, Merulina sp., and a Mussidae, but there was no significant difference between the controls and treatments in relation to the taxa found on each grid. The overall mortality rates of coral spat were not significantly different between treatments and controls. These results show that the soft coral S. flexibilis, known to release allelopathic toxins, can depress recruitment of scleractinian corals, and that this depression is exercised in a directional manner, depending upon the prevailing currents.