Chemical Ecology of Sponges on Caribbean Reefs

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Sponges are important components of benthic marine communities, and are dominant on contemporary Caribbean coral reefs. Organic extracts of their tissues have yielded a wealth of unusual chemical compounds that are not involved in primary metabolism, and their ecological functions are only beginning to be understood. This project will address the defenses of Caribbean sponges, a group whose taxonomy and chemistry is fairly well described. Because reef sponges are abundant, sessile, elaborate putative structural and chemical defenses, and are subject to grazing from generalist and specialist predators, they provide a useful group for testing fundamental hypotheses proposed by terrestrial ecologists about plant defensive mechanisms. The research has three primary objectives. The first is to extend investigations of sponge allelopathy to include sponge-coral interactions. By combining a modified gel-based field assay with the use of diving pulse amplitude modulated fluorometry, preliminary results have shown differential effects of sponge metabolites on coral photosynthesis and bleaching. This method will be used to test several hypotheses about interference competition between sponges and corals and the identities of the metabolites involved. The second objective is to test hypotheses regarding resource allocation by the two dominant classes of reef sponges: chemically defended species and those that are palatable to sponge-eating fishes. Preliminary results have demonstrated that the latter class heals wounds faster than the former, and grows faster in caging experiments in the field. This project will include comparisons of growth and reproductive output of several species in each class. The third objective is to investigate the importance of photosymbionts in the chemical defense and bleaching of the giant barrel sponge Xestospongia muta. The broader impacts of the project support and training for undergraduate and graduate students and for a Fulbright post-doctorate scholar, collaboration among scientists and students from six nations on three research cruises, and web-based outreach, including links on demography and bleaching of X. muta and a photographic key to the sponges of the Caribbean. The results will be useful in judging the general applicability of chemical defense theories derived from studies of terrestrial ecosystems, while advancing an understanding of the complex relationships among benthic invertebrates, their predators and their competitors in coral reef environments.