Variation in symbiont density is linked to changes in constitutive immunity in the facultatively symbiotic coral, Astrangia poculata

Scleractinian corals are essential ecosystem engineers, forming the basis of coral reef ecosystems. However, these organisms are in decline globally, in part due to rising disease prevalence. Most corals are dependent on symbiotic interactions with single-celled algae from the family Symbiodiniaceae to meet their nutritional needs, however suppression of host immunity may be essential to this relationship. To explore immunological consequences of algal symbioses in scleractinian corals, we investigated constitutive immune activity in the facultatively symbiotic coral, Astrangia poculata. We compared immune metrics (melanin synthesis, antioxidant production, and antibacterial activity) between coral colonies of varying symbiont density. Symbiont density was positively correlated to both antioxidant activity and melanin concentration. Our results suggest that the relationship between algal symbiosis and host immunity may be more complex than originally hypothesized and highlight the need for nuanced approaches when considering these relationships.