Sponges have not only the ability of rapidly overgrowing benthic reef organisms but they also release chemical compounds that can harm and kill other competitors [15,28,46,48]. Cytotoxic secondary metabolites produced by sponges may be able to inhibit the growth of other competing organisms by impairing their cell division and thus provide sponges with an advantage during competition for space on crowded coral reef substratum [54,55]. The bioactivity, especially the cytotoxicity, of sponge extracts seems to be a good proxy for their ability to overgrow corals in the field [56]. The four most bioactive sponges in the Spermonde Archipelago were also reported to cause necrosis of corals in more than 85% of interactions observed in situ [15]. Bioactive compounds are released through tissue contact, via sponge mucus or directly into the surrounding water causing bleaching and tissue necrosis in neighbouring corals thereby reducing their chances of survival [28,46,57]. Several sponges have already been identified to use allelopathy in order to inhibit the growth of other benthic organisms or were even able to cause bleaching and tissue necrosis in neighbouring corals [15,28,46,48,58].
