Project: Chemical ecology of sponges on Caribbean coral reefs

NSF Award Abstract:  

Sponges are now the dominant habitat-forming animals on most Caribbean coral reefs. Unlike corals and some macroalgae, sponges have uncalcified skeletons, and are less prone to effects of ocean acidification. A recently published demographic study of the giant barrel sponge on the Florida Keys reefs showed population increases by ~40% between 2000 and 2006. This renewal project would investigate the chemical ecology of Caribbean reef sponges, a group whose taxonomy and secondary metabolites are well described. Some reef sponges produce chemical defenses, while others are subject to grazing by fish predators. The collective community is found over a large biogeographic area where variable anthropogenic impacts permit the testing of fundamental hypotheses about ecosystem function, indirect effects, and resource allocation.

Intellectual merits: Previous NSF-funded research has transformed understanding of Caribbean coral reef ecosystems. A survey of chemical, structural and nutritional anti-predatory defenses of over 70 species of Caribbean sponges, followed by field experiments using natural populations of reef fishes, resulted in the isolation and identification of deterrent compounds from over 15 species. A series of manipulative experiments clearly demonstrated that sponge-eating fishes limit sponge distributions, and that parrotfishes are major spongivores, thereby overturning conventional ideas about effects of sponge-eating fishes on reef communities. Novel gel-based assays revealed differential allelopathic effects of sponge metabolites against other sponge and coral species. The ecosystem model for Caribbean reefs thus involves trophic and competitive interactions, predicting cascades and indirect effects known for other ecosystems.

Three primary objectives for testing the ecosystem model are to: (1) extend studies of top-down control of the sponge community. Guided by the World Resources Institute "Reefs at Risk" database, predictions and comparisons will be made of the community structure of sponges and their predators on overfished vs. well-protected reefs across sub-regions of the Caribbean. Parrotfish predation on sponges will be video recorded during food choice experiments on differently impacted reefs. Studies of allelopathic competitive interactions between sponges and corals (sponge metabolites on coral photosynthesis and bleaching) will continue using a modified gel-based field assay and diving-PAM fluorometry; (2) improve testing of the alternative hypothesis that bottom-up processes -- availability of picoplankton as food -- control reef communities. Predator-exclusion experiments will decouple effects of predation from sponge growth at picoplankton-rich and -poor, deep- and shallow-reef sites; (3) expand studies of sponge life history trade-offs in resource allocation between chemical defense, growth and reproduction. Differences in recruitment and succession will be examined among sponge communities of known age on artificial reef surfaces. This component builds on the recent discovery of sponge community succession on the deck of the Spiegel Grove shipwreck off Key Largo, FL, which strongly suggests a resource trade-off between chemical defenses and reproduction or growth.

Broader impacts: Renewal of this research program will provide (1) support and training for undergraduate and graduate students at a teaching-intensive, predominantly MS-level university (>68% of direct costs for student support), (2) collaboration between scientists and students from the US and abroad on three 2-week research cruises, (3) web-based outreach, including updated links on the demographics, bleaching, and chemical defenses of Caribbean sponges and further refinement of an easy-to-use photographic key to sponges of the Caribbean. Results of this project will be useful in judging the general applicability of chemical defense theories derived from studies of terrestrial ecosystems, while advancing understanding of the complex relationships between benthic invertebrates, their predators and their competitors in coral reef environments where the effects of global climate change and ocean acidification may be tipping the competitive balance toward non-calcifying organisms, such as sponges.