CHEMICALLY MEDIATED INTERACTIONS BETWEEN THE RED ALGA PLOCAMIUM-HAMATUM (RHODOPHYTA) AND THE OCTOCORAL SINULARIA-CRUCIATA (ALCYONACEA)

Interactions between the red alga Plocamium hamatum J. Agardh (Rhodophyta) and other benthic organisms including the alcyonacean soft coral Sinularia cruciata (Tixier-Durivault) were investigated on an inshore fringing reef environment in which P. hamatum was the dominant large fleshly alga. Field observations of sessile reef organisms including octocorals and sponges living in close proximity to P. hamatum revealed that varying degrees of tissue necrosis were suffered by the invertebrates when in physical contact with the alga. In order to establish whether the chemical constituents of the alga, especially chloromertensene, played a role in this necrosis, manipulative field experiments were carried out in the Pelorus Channel, Palm Island group (18-degrees-34'S; 146-degrees-29'E), North Queensland, Australia, in November and December 1988. The first experiment involved the relocation of healthy plants and soft corals into contact and non-contact situations on a mesh grid. In all cases of contact between P. hamatum and S. cruciata, the soft coral suffered tissue necrosis (n = 6, p = 0.0022). The second experiment had the same design, but involved the use of artificial "plants" both uncoated and coated with natural levels of chloromertensene, in contact with S. cruciata. In all cases of contact with coated treatments, necrosis was observed in S. cruciata (n = 4, p = 0.025). In cases where uncoated artificial fronds were placed in contact with soft corals, S. cruciata showed minor abrasion effects, but no appreciable necrosis. Coated treatments were not fouled by epiphytes during the experiment and were not consumed by predators. Uncoated treatments were rapidly reduced in size by predation and any remaining material was biofouled. These experiments thus demonstrated that the deleterious effects observed in soft corals in the field were caused by contact with the alga P. hamatum, that these effects were indeed chemically mediated by chloromertensene, and that physical contact without chemical intervention caused no such deleterious effects. This is the first experimental evidence which conclusively demonstrates allelopathy between an alga and other marine organisms and identifies the compound responsible for the observed allelopathic effects.