Diversity in skeletal architecture influences biological heterogeneity and Symbiodinium habitat in corals.

Scleractinian corals vary in response to rapid shifts in the marine environment and changes in reef community structure post-disturbance reveal a clear relationship between coral performance and morphology. With exceptions, massive corals are thought to be more tolerant and branching corals more vulnerable to changing environmental conditions, notably thermal stress. The typical responses of massive and branching coral taxa, respectively, are well documented; however, the biological and functional characteristics that underpin this variation are not well understood. We address this gap by comparing multiple biological attributes that are correlated with skeletal architecture in two perforate (having porous skeletal matrices with intercalating tissues) and two imperforate coral species (Montipora aequituberculata, Porites lobata, Pocillopora damicornis, and Seriatopora hystrix) representing three morphotypes. Our results reveal inherent biological heterogeneity among corals and the potential for perforate skeletons to create complex, three-dimensional internal habitats that impact the dynamics of the symbiosis. Patterns of tissue thickness are correlated with the concentration of symbionts within narrow regions of tissue in imperforate corals versus broad distribution throughout the larger tissue area in perforate corals. Attributes of the perforate and environmentally tolerant P. lobata were notable, with tissues ∼5 times thicker than in the sensitive, imperforate species P. damicornis and S. hystrix. Additionally, P. lobata had the lowest baseline levels of superoxide and Symbiodinium that provisioned high levels of energy. Given our observations, we hypothesize that the complexity of the visually obscured internal environment has an impact on host-symbiont dynamics and ultimately on survival, warranting further scientific investigation.