Importance of philopatry and hydrodynamics in the recruitment of bioeroding sponges on Indonesian coral reefs

Increasingly frequent large-scale coral mortality events are occurring across the globe, leading to a rise in available reef substrata and promoting an increase in the abundance of other benthic taxa. One such group are bioeroding sponges, which can benefit considerably from reef degradation. However, the occupation of new substrates is partially reliant upon larval recruitment, and currently little is known about the factors affecting bioeroding sponge recruitment. In this study we investigated the potential drivers of bioeroding sponge recruitment using a 2-year deployment of experimental calcareous substrates across seven reefs in the Wakatobi region of Indonesia. Recruitment was observed for five bioeroding sponge species, namely Cliona orientalis, Cliothosa cf. aurivillii, Cliothosa hancocki and two presently unidentified brown clionaids, Cliona aff. viridis sp. A and Cliona aff. viridis sp. B. Recruits were present on 69% of the experimental substrates but had a low mean (+/- s.e.m.) spatial coverage of just 0.42 +/- 0.13%. Total recruitment and species assemblage structure were correlated with local adult abundance, water flow and substrate cues. Our results suggest that any proliferation of bioeroding sponges on newly available substrate following coral mortality is likely to be conditional on local adult abundance and hydrodynamics.