TOXIC EFFECTS OF ALCYONACEAN DITERPENES ON SCLERACTINIAN CORALS

Soft corals like Sinularia flexibilis (Quoy and Gaimard) and Lobophytu m hedleyi (Ehrenberg) inhibit growth and produce tissue necrosis in ne ighbouring scleractinian corals. Chemical analysis has shown that thes e soft corals exude terpenes, toxic allelomones which can kill certain hard corals in the vicinity. This study investigates the stepwise pro cess by which soft coral diterpenes induce cytological damage in scler actinian corals. Small branchlets of Acropora formosa (Dana) and Porit es cylindrica (Dana) were exposed to concentrations between 5 and 20 p pm of diterpenes derived from S. flexibilis and L. hedleyi, Concentrat ions higher than 5 ppm caused the initial expulsion of zooxanthellae w hich was followed by release of nematocysts. These processes were acco mpanied by inhibition of polyp activity, necrosis and ultimate death o f coral polyps. The expelled cells were viable supporting the suggesti on that the diterpenes act as membrane active, lipolytic agents. The l oss of nematocysts and zooxanthellae from a heterotrophic scleractinia n cord essentially removes its ability to procure food from the enviro nment in the form of phyto and zooplankton, to defend itself against c ompetitors for space and predators, and to obtain carbohydrates and ot her organic molecules fixed by photosynthetic symbionts.