Endoscopic exploration of Red Sea coral reefs reveals dense populations ofcavity-dwelling sponges

Framework cavities are the largest but least explored coral reef habitat(1) . Previous dive studies of caverns, spaces below plate corals, rubble and a rtificial cavities(1-3) suggest that cavity-dwelling (coelobite) filter-fee ders are important in the trophodynamics of reefs(2,4,5). Quantitative comm unity data are lacking, however, as the bulk of the narrow crevices interla cing the reef framework are inaccessible to conventional analysis methods(6 ). Here we have developed endoscopic techniques to explore Red Sea framewor k crevices up to 4 m into the carbonate rock, revealing a large internal su rface (2.5-7.4 m(2) per projected m(2) reef) dominated by encrusting filter -feeders. Sponges alone provided up to 60% of coelobite cover, outweighing epi-reefal filter-feeder biomass by two orders of magnitude. Coelobite comm unity filtration removed more than 60% of the phytoplankton in the course o f its less than 5-minute passage through the crevices, corresponding to an uptake of roughly 0.9 g carbon m(-2) d(-1). Mineralization of the largely a llochthonous organic material is a principal source of nutrients supporting coral and algal growth. The supply of new material by coelobites may provi de a key to understanding the 'coral reef paradox'-a rich ecosystem thrivin g in nutrient-poor water.