Background/Question/Methods Density-dependent survivorship is an important structuring force in marine communities; for sessile organisms, this should manifest as explicit distance-dependence. We sought to determine whether survivorship of juvenile reef corals depends on distance to the nearest conspecific adult and whether this effect might be mediated by species-specific pathogens, two predictions of the Janzen-Connell hypothesis. Over five spawning periods, we reared juveniles of Montastraea faveolata, a dominant Caribbean reef-building coral. To measure survivorship, we conducted field experiments with settled coral recruits and laboratory experiments with swimming coral larvae. To examine microbial effects, we manipulated the microbial environment in the lab using filters and antibiotics and we collected complementary profiles of reef water microbes surrounding adult coral heads.

Results/Conclusions Montastraea faveolata juveniles exhibited species-specific, distance-dependent survivorship in both field and lab experiments. Both experimental manipulation and observational data show that microbial activity is a likely driver of this effect. The assembly of coral communities is therefore governed not only by larval supply on a scale of meters to kilometers but also by microbial ecology on a scale of millimeters to meters. Because they affect survivorship during a coral's most sensitive life phase, microbes—and human alteration of microbial communities—have important consequences for the dynamics of both healthy and recovering coral reefs.