PAWLIK, J.R.; Univ. of North Carolina at Wilmington: Adaptation and optimization of chemical defenses in coral reef sponges

Marine chemical ecologists investigate the functional significance of secondary metabolites present in marine organisms within a theoretical framework that has been borrowed from earlier terrestrial studies, particularly plant-insect interactions. Foremost among models predicting intraspecific allocation of chemical defenses is optimal defense theory, which predicts that metabolites will be differentially allocated to susceptible parts of a prey organism, or that metabolite production will occur quickly in response to predation. Our studies of the chemical defenses of Caribbean sponges have yet to uncover intraspecific metabolite distributions that conform to optimization theory. For species that we have studied to date, there was no evidence that deterrent compounds were concentrated in the surface tissues of the sponge, or that chemical defense could be induced by simulated predation. The hypothesis that some sponge species rely on damage-induced activation of chemical defenses in which high molecular weight metabolites undergo rapid, enzyme-mediated transformations following tissue damage, a process also called “biotransformation,” could not be demonstrated to occur in laboratory and field time-course experiments. Past observations of transformation may be the result of differential tissue extraction efficiency, hydrolysis from insoluble precursors, or the heterogenous distribution of metabolites in sponge tissue. Patterns of metabolite use by marine sponges may not conform to theory for many reasons, foremost being that resources for metabolite production are not limiting. Alternatively, metabolites may play multiple defensive roles, and may be subject to conflicting evolutionary trajectories.