Anti-predatory effects of organic extracts of 10 common reef sponges from Zanzibar

Predation is a key factor influencing the distribution and community composition of sponges and other benthic organisms. The ability to produce deterrent secondary metabolites may partially explain the high abundances of sponges on coral reefs worldwide. Nonetheless, studies investigating sponge abundances and chemical ecology are rare, particularly in the Western Indian Ocean. Therefore, this study assessed whether predation is a key factor in structuring the sponge community on reefs around Zanzibar by testing whether chemical defence potential correlates with sponge abundance. Sponge biodiversity and abundances (surface area) were determined, and the palatability of the most abundant sponge species was tested in laboratory feeding assays with the spongivorous pufferfish Canthigaster solandri. Sponges were abundant on the reefs on Zanzibar accounting for 4.8 +/- 3.8 and 7.5 +/- 1.7% of the benthic cover at 5 m and at 10 m depth, respectively. In the feeding assay, three sponges deterred feeding by C. solandri. However, the presence of feeding deterrent compounds in sponges did not correlate with their abundance on the reef. Low predatory fish abundances likely resulted in a high prevalence of chemically undefended species. Thus, chemically undefended sponges dominate the reef at Bawe Island, Zanzibar, subjecting reef-building corals to a higher competitive pressure.