Larvae from three Caribbean corals settle differently in response to crustose coralline algae and their bacterial communities

Certain species of crustose coralline algae (CCA) provide settlement cues for larvae of scleractinian coral species. However, it remains largely unknown whether larval settlement is mediated by compounds produced by the CCA itself, by compounds produced by the microbial community associated with the algal surface, or by interactions between the two. In the present study, settlement experiments were implemented to examine the possible roles of CCA and/or their associated microorganisms in coral settlement. Five species of CCA commonly found in the Florida Keys (USA), with or without antibiotic treatment (30 mg l -1 each of penicillin, streptomycin, and gentamycin) to reduce their associated bacterial communities, were used in settlement experiments with larvae from 2 species of broadcast spawning corals: Dendrogyra cylindrus and Orbicella faveolata . Larvae from an additional broadcast spawning coral, Colpophyllia natans, were used to assess settlement preferences among the CCA species. Sequencing of the V4 region of the 16S rRNA gene permitted molecular characterization of the CCA-associated prokaryotic communities to examine whether coral larval behavior was linked to the presence/absence and/or differential abundance of particular taxa. Results showed that antibiotic treatment reduced the culturable CCA-associated prokaryotic community by at least 97.6%, eliciting different responses from the coral larvae based on the coralline species tested, and that coral larvae showed varying degrees of specificity for CCA. Given the dramatic decline of coral reefs worldwide, these results highlight the potential of CCA to accelerate reef recovery as well as informing conservation efforts about the variability in settlement preferences of larvae from different corals.