Bacterial suspension feeding by clionid sponges

The changes in seawater quality that are associated with eutrophication due to increased antropogenic input can have a major impact on marine communities. It is hypothesised that nutrient enrichment indirectly enhances the growth and production of bacteria, thereby stimulating the growth of organisms that feed on microbes, such as sponges. Increased infestation of corals by boring sponges accelarates bioerosion and may result in a net degradation of the reef. To gain more insight in the trophic relationship between the microbial community in the water column and benthic reef communities, uptake rates of three species of Clionidae were investigated. Experiments were performed in situ, with use of enclosures. Bacterial numbers were determined with a direct count method using acridine orange staining and epifluorescence microscopy. The results show that C. lampa, C. laticavicola as well as C. vermfera are effective bacterial suspension feeders, with the capability to adapt their feeding strategies to changing densities of food particles over a short time span. We have indications that the optimal clearance rates are different between the species. C.lampa is potentially capable of maintaining a higher clearance than C.laticavicola and C.vermfera. The results furthermore indicate that C.lampa and C.vermfera have a higher retention efficiency for picoplankton than for nanoplankton. No relationship was found between the clearance rates and biomass of the sponges. We showed that the species are capable of efficient filtration of enhanced bacterial densities. Considering their responsiveness to changing bacterial densities together with the destructive qualities these species have, we suggest that clionid sponges potentially form a strong link between changes in the water column microbial population and the global deterioration of coral reefs.