RAFTING OF TROPICAL MARINE ORGANISMS ON BUOYANT CORALLA

Coral reef species are amongst the most widespread of all organisms, via dispersal of teleplanic larvae in oceanic surface currents and transport of adults on floating rafts. This note describes the rafting of tropical marine organisms on a buoyant skeleton of the reef coral Symphyllia agaricia. When found drifting on the Great Barrier Reef, the corallum was covered in coralline and filamentous algae and supported goose barnacles, decapod crustaceans, pearl and reef oysters, gastropods, foraminiferans and bryozoans. The size structures of the rafted species suggested that the corallum had been floating for several months. Bouyancy was due to gas trapped within the coral's septal chambers. The gas was qualitatively similar to air but differed quantitatively in composition, probably the result of bacterial respiration. Coralla of several scleractinian species can float following desiccation after beaching during storms. Whilst uncommon, drifting coralla may have been of considerable significance as agents of dispersal over geological time, as biogenic carbonates are the usual settlement substrata of most sessile reef species.