Examining symbiont selection and polyp connectivity in Astrangia poculata

Given the recent acceleration in global coral reef decline, there is a growing interest in understanding resilient corals. Increasing ocean temperatures cause a breakdown of the symbiotic relationship between corals and the dinoflagellate Symbiodinium, leading to expulsion and eventually colony mortality due to lack of nutrition normally provided by symbionts. Astrangia poculata is distinctive in that it can survive in both symbiotic and aposymbiotic states, unlike most tropical corals that have an obligate relationship with Symbiodinium. It is therefore an exceptional model system to understand this symbiosis. Our study focuses on the reintroduction of Symbiodinium following a bleaching event and the connectivity between polyps in a colony. In this study, we reintroduced symbionts in a single polyp of naturally and chemically bleached colonies of A. poculata using two different clades of cultured Symbiodinium, as well as tissue from a symbiotic A. poculata colony. Image analysis was used to evaluate symbiont density following reintroduction. We found that successful reintroduction of symbionts was independent of how the coral was bleached and the clade of symbiont used, showing that tissue reinfection is the most effective method. This study demonstrates that reintroduction of symbionts is possible in a laboratory setting for A. poculata. We also found that there was a lack of polyp connectivity in grown A. poculata colonies as well as connectivity with surrounding polyps. This suggests that A. poculata polyps lose connectivity after development, unlike those in tropical corals like Acropora and Porites, species that maintain connectivity as developed polyps. These results offer insight into the symbiosis and colonial interaction of A. poculata and how they relate to wholly tropical corals, providing a deeper understanding of resilient corals in the face of climate change.