ALLELOPATHIC INTERACTIONS BETWEEN SPONGES ON A TROPICAL REEF

Competition for space among organisms on tropical reefs has often been hypothesized to be mediated by allelopathic interactions, but the secondary metabolites involved in these interactions have rarely been identified. On Guam, the sponge Dysidea sp. overgrows the sponge Cacospongia sp. and causes necrosis. Using field assays, we tested the effects of crude organic extracts and a major sesquiterpene isolated from Dysidea, 7‐deacetoxyolepupuane, on Cacospongia and on the production of the major terpenoid metabolites scalaradial and desacetylscalaradial by Cacospongia. We also tested whether the amounts of organic compounds produced by Dysidea differed in the presence or absence of Cacospongia. To determine whether 7‐deacetoxyolepupuane had another ecological function, we tested the compound as a predator deterrent. In field experiments, crude extracts of Dysidea and pure 7‐deacetoxyolepupuane both caused necrosis in Cacospongia when they were incorporated into agar strips and placed in contact with Cacospongia for 7 d. Organic extract, scalaradial, and desacetylscalaradial concentrations in Cacospongia were not affected by the overgrowth of Dysidea. However, a greater quantity of organic extract (but not scalaradial or desacetylscalaradial) was found in portions of Cacospongia covered by agar strips containing Dysidea organic extracts than in portions of Cacospongia covered by control agar strips. The production of 7‐deacetoxyolepupuane by Dysidea occurring on rock substrates and on Cacospongia did not differ; thus, the production of this compound is not induced by the presence of competitors. In addition to its role in competition, 7‐deacetoxyolepupuane deterred predation by a spongivorous fish, illustrating the multiple ecological roles that a single secondary metabolite may play.