Mutualistic symbiosis with ophiuroids limited the impact of the Deepwater Horizon oil spill on deep-sea octocorals

Deep-water corals form structurally complex biological habitats in the deep-sea that are generally associated with a diverse fauna. Yet, little is known about the effect of symbionts on coral resilience to natural or anthropogenic impacts. This study focused on the influence of the ophiuroid symbiont Asteroschema clavigerum on the resilience of its octocoral host Paramuricea biscaya after the Deepwater Horizon oil spill in the Gulf of Mexico. Corals were imaged between 2011 and 2014 at 4 sites, 3 of which were impacted by the spill. Each colony was digitized to quantify the impact on corals. We developed a method to define an area under the influence of ophiuroids for each coral colony. The level of total visible impact, as well as recovery, was then compared within and outside this area. For the majority of colonies, recovery from visible impact and hydroid colonization was negatively correlated with distance from the ophiuroid. Total visible impact was lower within the area influenced by ophiuroids, and branches within this area were more likely to recover. These results indicate that P. biscaya benefits from its association with A. clavigerum, likely through the physical action of ophiuroids removing material depositing on polyps, and perhaps inhibiting the settlement of hydroids. Although the beneficial role of the ophiuroids was demonstrated on corals affected by an oil spill, we suggest that these benefits would also extend to corals in environments exposed to natural sedimentation events, perhaps allowing the corals to live in environments where sedimentation would otherwise limit their survival.