CHEMICAL WARFARE AND OVERGROWTH IN TROPICAL SPONGES

Mikel A. Becerro, Robert W. Thacker, Wilfred A. Lumbang and ValerieJ. Paul
University of Guam Marine Laboratory, UOG Station, Mangilao, Guam 96923,USA

Competition for space among organisms on tropical reefs has often beenhypothesised to be mediated by allelopathic interactions, but the secondarymetabolites involved in these interactions have rarely been identified.On Guam, the sponge Dysidea sp. overgrows the sponge Cacospongiasp. and causes necrosis. Using field assays, we tested the effects of crudeorganic extract and a major sesquiterpene isolated from Dysidea,7-deacetoxy-olepupuane, on Cacospongia and on the production ofthe major terpenoid metabolites scalaradial and deacetylscalaradial byCacospongia. We also tested whether the amounts of organic compoundsproduced by Dysidea differed in the presence or absence of Cacospongia,and whether 7-deacetoxy-olepupuane acts as a predator deterrent. In fieldexperiments, crude extracts of Dysidea and pure 7-deacetoxy-olepupuanecaused necrosis in Cacospongia when they were incorporated intoagar strips and placed in contact with Cacospongia for seven days,but scalaradial and deacetylscalaradial concentrations in Cacospongiawere not effected. The production of 7-deacetoxy-olepupuane by Dysideaoccurring on rock substrates and on Cacospongia did not differ;thus, the production of this compound is not induced by the presence ofcompetitors. In addition to its role in competition, 7-deacetoxy-olepupuanedeterred predation by a spongivorous fish, illustrating the multiple ecologicalroles that a single secondary metabolite may play.