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Sponges are among the dominant benthic organisms on coral reefs, representing important spatial competitors for reef-building corals. Coral reefs have experienced drastic declines in coral cover and corresponding increases in the abundance of other spatial competitors, such as macroalgae, corallimorpharians and sponges, due to a combination of global and local stressors. The ability of sponges to chemically defend themselves against predators, microbes and other competitors may partially explain their high abundance on reefs worldwide. Nonetheless, studies investigating sponge abundance and chemical ecology are rare, particularly in the Western Indian Ocean, which is considered a hotspot of coral and sponge biodiversity. Thus, this thesis is the first study that provides insights into the chemical ecology of sponges from the Western Indian Ocean. The thesis consists of a general introduction, three chapters investigating in detail different aspects of chemical defence mechanisms in sponges from Zanzibar and a general discussion. In Chapter 1 I assessed the benthic community composition of the reef at Bawe, an island on Zanzibara s West Coast. This reef was dominated by reef-building corals, but sponges also represented a diverse and abundant component of the reef fauna. Moreover, laboratory experiments were conducted to investigate if predation was a key factor in structuring the sponge community. Findings revealed that the reef was dominated by palatable rather than chemically defended sponge species, demonstrating a lack of predatory control on the sponge community likely due to overfishing. In the absence of predation, palatable sponges could potentially undergo uncontrolled growth in the future and thus subject reef-building corals to greater competitive pressure. Chapter 2 focused on the antimicrobial and cytotoxic activities of secondary metabolites extracted from the most abundant sponges at the reef around Bawe. The experiments revealed that the most abundant sponges were remarkably well defended against co-occurring marine bacteria and in particular against potential pathogens. Moreover, the majority of the sponge extracts also displayed cytotoxic activities. Metabolites with either antimicrobial or cytotoxic properties can provide sponges with a competitive advantage over corals. Antimicrobial compounds could alter the coral microbiota while cytotoxic compounds are able to impair the cell division of corals. Consequently, the potential allelopathic properties of sponge crude extracts from the three most abundant and bioactive sponges were further examined in field experiments under natural conditions in Chapter 3. These experiments showed that the extracts of all three sponges possessed allelopathic compounds that adversely affected the photosynthetic efficiency of the corals symbiotic zooxanthellae. Despite this, experiments with live sponge fragments were unable to detect significant impairments on the physiology of corals. However, the presence of allelopathic compounds, and their significantly negative effect on coral photo-efficiency, suggests that allelopathy by sponges does play a role in spatial competition with corals. As a result, sponges might be able to exert negative effects on the corala s fecundity, their reproduction or even their associated microbiome making corals more vulnerable towards further natural or anthropogenic disturbances and pathogenesis. Collectively, the present study demonstrated that sponges on reefs in Zanzibar are serious spatial competitors against reef-building corals. The increased sewage input in combination with other local stressors, such as destructive fishing practices or damage to the reef through tourism activities, will most likely result in more frequently occurring sponge-coral interactions. Thus, management strategies for Zanzibarian reefs should focus on minimizing anthropogenic stressors, like the establishment of a sewage water treatment facility to minimize stress on the benthic community, in order to reduce the risks to coral reef health. Fisheries regulations should also be enforced as an important component of coral reef management plans in order to restore healthy herbivorous as well as spongivorous fish populations on the reefs, which limit the proliferation of competing macroalgae and sponges.