Patterns of surface lesion recovery in the Northern Star Coral, Astrangia poculata

Corals regularly experience partial mortality due to grazing, sedimentation, disease, and abrasion. Because a colony's ability to heal such surface lesions can reflect its overall health, recovery from artificial wounds has been posed as a potential in situ measure of coral status and resilience. In this study, the response of a temperate, facultatively symbiotic coral, Astrangia poculata, to wounds of varying size and shape was measured, along with the influence of Symbiodinium psygmophilum. Because most tropical corals host an obligate symbiosis, the impact of the symbionts on lesion recovery cannot be quantified separately from the hosts. Here, symbiotic and aposymbiotic A. poculata colonies were subjected to lesions representing 25%, 50%, and 75% total colony loss, with varying wound shape. Colonies were monitored for recovery, mortality of living tissue, photosynthetic efficiency and approximated chlorophyll density. Recovery was observed to occur primarily from within the lesion site, demonstrating a 'Phoenix effect' healing pattern whereby recovery stems from tissue remnants in the wound area. Recovery was best explained by an interaction effect between symbiotic state (symbiotic versus aposymbiotic) and the amount of residual tissue visible after wounding. Per unit area of residual tissue, healing was greater in symbiotic colonies. Symbiotic colonies also exhibited higher estimates of photochemical efficiency and approximated chlorophyll density, likely contributing to their enhanced healing capacity. These results support previously documented reductions in wound recovery among bleached corals, and suggest that symbiont loss itself, rather than associated stress or physiological disruption contributes to the diminished healing capacity of bleached colonies. Tissue mortality in un-wounded colony portions was greatest among symbiotic corals exposed to 50% colony loss amassed over two wounds, suggesting that corals with multiple injuries may experience greater tissue loss compared to single-wounded colonies, which can help inform methods for wound proxy metrics in the field. Dissected decalcified A. poculata colonies revealed deep (~ 4-6 mm) corallite and polyp penetration, suggesting that surface lesions in this species are likely to leave tissue remnants buried within the skeleton. As such, it appears that A. poculata and corals with similar morphologies are predisposed to 'Phoenix effect' recovery, since most natural lesions would not fully penetrate the full depth of their corallites.