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.