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Secondary metabolites from marine invertebrates, such as sponges, have diverse ecological roles and may be used in anti-predation, anti-fouling and spatial competition. The thin encrusting sponge Iotrochota sp., found on the Great Barrier Reef, Australia, contains several novel indoles that may aid in spatial competition. To examine whether levels of surrounding competition affect the toxicity of Iotrochota sp., 24 sponges from Salamander Reef, Central Great Barrier Reef, subject to either high or low levels of surrounding competition, were sampled. Tissue samples were taken from areas where the sponge was in direct competition with neighbouring sessile invertebrates, as well as from areas where competition was less obvious. Compounds from the tissue samples were extracted and tested in both cytotoxicity and anti-microbial assays. However, neither cytotoxicity nor anti-microbial activity varied significantly …