Competition between alcyonacean and scleractinian corals at Britomart Reef, central Great Barrier Reef

Experiments were performed on Britomart Reef in the central region of the Great Barrier Reef, in a large depression behind the reef crest, 50-60 m in diameter and 11 m deep, which possessed a well-developed coral community. Initially, observations of interactions between alcyonacean (soft) and scleractinian (hard) corals were made in shallow water, where both occurred in close proximity. Observations involved 7 genera of soft corals from 3 families and 9 genera of hard corals. The effects of soft corals on hard corals were classified as lethal (local mortality), sublethal (stunting or growth inhibition), or no effect. The potential for contact between polyps of different colonies was also assessed. The effects of hard corals on soft corals was not assessed.In August 1980, an experiment was initiated using three species of soft corals (Lobophytum pauciflorum, Sinularia pavida and Xenia sp. aff. danae) and two species of hard corals (Pavona cactus and Porites andrewsi (= Porites cylindrica)). Small colonies of soft coral were removed from the reef and labelled. One soft coral colony of each species was moved to natural spaces within each of five large stands of Pavona and five stands of Porites. Several specimens of each species of soft coral were moved to a nearby area, which was naturally clear of scleractinian corals as a control. For each of the stands of scleractinian coral, small healthy areas were marked and designated as controls. Direct observations of interactions were made in November and December 1980 and March 1981. Interspecific interactions were recorded as the presence or absence of local colony mortality, as a function of contact with versus mere proximity to a neighbouring colony. Sublethal effects (eg stunting of growth) were also noted. Any relocated soft coral which disappeared during the course of the study was replaced accordingly. In a second experiment, initiated on the 15th June 1981, two species of scleractinian corals (Pavona cactus and Porites andrewsi (= Porites cylindrica)) and three species of soft corals, chosen for their range of ichthyotoxicity were used. The soft coral species were: Sarcophyton ehrenbergi (containing substantial concentrations of diterpenes); Nephthea brassica (intermediate and variable in toxin content); and Capnella lacertiliensis (a generally non-toxic genus). Small pieces of reef substratum bearing suitably sized colonies of each soft coral (~10-15 cm in height and 5-10 cm in diameter) were excised from the reef. Two colonies of each were labelled and relocated to natural spaces within each of five stands of Porites and five stands of Pavona. The soft corals were placed so that parts of each soft coral colony was in contact with the scleractinian, while other parts were clearly not. Eight specimens of each soft coral species were moved to nearby areas without scleractinian coral, as a control. Two healthy areas of each scleractinian coral stand were also used as controls. Direct observations of interactions between the alcyonacean and scleractinian corals were made over the next 8 months, after 3, 8, 18, 28, and 32 weeks. A number of categories of interspecific interactions were recorded. Soft corals were examined for local mortality, as evidenced by tissue necrosis. Local mortality was also recorded as a function of 'contact' versus 'non-contact', with a neighboring colony. Soft corals were also examined for evidence of defensive secretions or actual movement or displacement of the colony through bending and regrowth. Local mortality of the scleractinian corals was assessed along with overgrowth of the colony by soft corals. Sublethal effects were also noted. Any relocated soft corals which disappeared during the course of the study was replaced.
 Manipulative field experiments were conducted to determine:- whether soft corals (Alcyonacea) can be effective competitors for space against hard corals (Scleractinia) by causing localized mortality in the latter- whether mortality can be caused either by direct contact between soft and hard corals or can be effected from a distance through the water, implicating allelopathic exudates as a causal factor- whether competitive abilities of alcyonacean corals varies in a species specific manner- whether susceptibility of scleractinian corals to the deleterious effects induced by alcyonacean corals also varies in a species-specific manner.- whether hard corals (Scleractinia) can be effective competitors for space against soft corals (Alcyonacea) by causing localized mortality in the latter.

Organization: Australian Ocean Data Network