From the Florida Keys to the Great Barrier Reef : exploring the feeding ecology of tropical marine sponges

Many marine sponges have developed strong associations with symbionts that live within the tissue of the sponge. One of the proposed benefits that the symbionts provide for the host is energy. That is, the symbionts add to the feeding repertoire of the sponge. For example, we have shown that zooxanthellae (Symbiodinium sp.) translocate photosynthetically derived carbon from the symbiont to the host sponge. The evolutionary implications of having zooxanthella-derived energetic inputs on a host sponge‟s heterotrophic feeding behavior are unknown. Less is known about the role that extracellular microbial symbionts play in sponge trophic status, though data indicate that they play an important role in sponge feeding. To complicate matters, we know that these symbiotic associations can be affected by environmental conditions, though we no little about how those changes affect sponge feeding. Climate change currently represents the most significant and increasing threat to coral reef ecosystems worldwide, putting sponges and their symbiont communities at risk. This study aims to explore the effects of symbiosis on sponge trophic status as well as the impacts of thermal stress on sponge feeding behavior. In the first part of the study, energy budgets were calculated for four clionaid species (three that harbor zooxanthellae and one that does not) found at varying depths in the Florida Keys. Cliona varians forma varians, a zooxanthellate sponge found in very shallow, high-light habitats, had a significantly lower carbon intake rate than the three zooxanthellate sponges found in deeper habitats. C. delitrix, an azooxanthellate sponge, had a lower carbon intake rate than zooxanthellate sponges at the same depth. The second part of the study investigated the effect of thermal stress on filtration efficiencies and choanocyte chamber characteristics in Rhopaloides odorabile, a Great Barrier Reef sponge. The threshold for normal sponge feeding behavior was 31oC, at which filtration efficiency on heterotrophic bacteria, volume flow rate, and choanocyte chamber density and size were all reduced. There was also evidence that R. odorabile is able to recover from short-term exposure to thermal stress. As efficient filter feeders, sponges are important components of the benthic environment, and it is essential that we understand how their feeding ecology responds to changes in temperature in order to develop improved conservation and management strategies for sponge communities worldwide