Sponge communities found in Caribbean mangroves are typical to this habitat: partly endemic and very distinct from sponge communities on nearby reefs. A tradeoff between resistance to competitors and predators appears to influence success of individual sponge species in mangrove habitats. We speculate that differences in the symbiotic microbial communities may partly be responsible for these differences, as partial degradation of recalcitrant compounds by tannin-degrading microorganisms may enhance palatability and facilitate dissolved organic matter (DOM) assimilation in the presence of high concentrations of tannins, thereby improving their competitive capabilities. We tested tannase activity and ability to degrade mangrove-derived DOM in a random set of sponge species collected from mangrove roots in Curaçao and adjacent reefs. Our results suggest that sponges commonly associated with mangrove roots contain bacteria that are capable of degrading mangrove-derived DOM, while bacterial communities associated with sponges that are more typical to reef environments appear less proficient in degrading mangrove-derived DOM. Host specificity of bacterial endobionts capable of degrading mangrove-derived DOM and the presence of high concentrations of recalcitrant organic compounds may lead to ecological separation between mangrove and reef sponge communities. © 2010 Rosenstiel School of Marine and Atmospheric Science of the University of Miami.