Juvenile octocorals acquire similar algal symbiont assemblages across depths

Establishment of the coral–algal symbiosis begins during early ontogeny when juveniles acquire a mix of algae from their environment that often differs from the adults’ algal assemblages. Despite the importance of the type of Symbiodiniaceae to this symbiosis, it is largely unknown how coral host identity and environment affect symbiosis establishment and is affected by the genetic composition of the symbionts. Here, we reciprocally transplanted planulae of the octocoral Rhytisma fulvum (Forskål, 1775) across depths and monitored the algal assemblages in the developing juveniles for 11 months. We then compared these to adult assemblages using ITS2 metabarcoding. Juveniles were consistently dominated by Symbiodinium, in addition to multiple Cladocopium species, which shifted in dominance with the juvenile age but maintained high similarity across depths. The type of Symbiodiniaceae environmentally available thus likely contributes to the algal symbionts that are initially acquired, while host identity may play a significant role in selecting for symbionts that are maintained during juvenile development.