Coral recruitment is impacted by the presence of a sponge community

As coral cover has declined on Caribbean reefs, space has become occupied by other benthic taxa, including sponges, which may affect the recruitment of new corals, thereby affecting the ability of reefs to recover to coral-dominated states. Sponges may inhibit coral recruitment by pre-empting potential recruitment space, overgrowing recruits, or through allelopathy. This study examined coral recruitment across six coral reef sites surrounding St. Thomas, US Virgin Islands, and the impact of one species of sponge and the sponge community as a whole on coral recruitment. To test the effect of a single species of sponge on coral recruitment, fragments of living or non-living Aplysina cauliformis were attached to terracotta recruitment tiles and deployed at all six sites, along with unoccupied tiles as controls. At two of the sites, a community-level experiment consisted of deploying recruitment tiles in 1m(2) plots that were either cleared of the entire sponge community or control plots where no sponges were removed. Recruitment rates showed a consistent difference among sites over multiple years and experiments. Results of the species-specific experiment showed that the proximity of live or dead A. cauliformis did not affect coral recruitment. However, results of the community-level experiment found greater coral recruitment rates in plots cleared of sponges, suggesting that the presence of the sponge community negatively affected coral recruitment. This study is one of the first to experimentally test and find a significant impact of sponges on coral recruitment, and highlights the need for additional research in this area.