RUI: Assessing the Chemical Defenses of Caribbean Invertebrates

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Sponges, gorgonians, and ascidians are important components of benthic marine communities, particularly coral reefs. Organic extracts of their tissues have yielded a wealth of unusual chemical compounds that are not involved in primary metabolism, and their biological functions are mostly unknown. One commonly held theory is that these compounds are distasteful to potential predators, but they may also protect from fouling or overgrowth. This study is a continuation of a very successful RUI research program designed to assess the defenses of Caribbean sponges, a group whose taxonomy and chemistry is fairly well described. Because reef sponges, gorgonians and ascidians 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.Investigations of antipredatory structural and chemical defenses will be extended from the previous study to include Caribbean gorgonians and ascidians, using the successful techniques previously applied to sponges. In particular, optimal defense theory will be tested with ascidians, which elaborate both inorganic and organic chemical defenses. Metabolites of invertebrate species that have yielded deterrent crude extracts will be identified by bioassay-directed fractionation, resulting in a better understanding of the structure-function relationships of chemical defenses. Intraspecific patterns of metabolite expression will be examined to determine whether they conform to optimal defense theory. Hypotheses regarding resource allocation between defensive chemistry and growth, reproduction, and regeneration will be tested by manipulating chemically defended and non-defended reef sponges in the field. The hypothesis that fish predation limits sponge distributions on reefs will be tested by comparing transect data from reefs where sponge-eating fishes are abundant (Bahamas, Florida Keys) to reefs where fish predators are rare due to natural (Bermuda) or anthropogenic causes (Jamaica, Barbados). The hypothesis that sponge secondary metabolites function as antifouling and anti-overgrowth defenses will be further tested in the field and laboratory with gel-based assays, and these techniques will be used to isolate the active metabolites. Field experiments will be conducted to determine the rates at which active metabolites are expressed by sponges onto their surfaces or into seawater.Overall, this research program will continue to develop one of the most systematic and complete investigations of chemical defenses of tropical invertebrates, while at the same time providing important educational opportunities for students at an RUI institution. The results will be useful in judging the general applicability of chemical defense theories derived from studies of terrestrial ecosystems, while advancing our understanding of the complex relationships between benthic invertebrates, their predators and their competitors in coral reef environments.