Skeletal structure and adaptive mechanisms of corals inhabiting a mangrove environment

Abstract Coral skeletons form the foundational framework of coral reef ecosystems but are threatened by climate change stress. Under sub-optimal conditions, skeletal calcification rates decrease, and corals have been observed to form more porous skeletal structures, raising concerns of sublethal effects under future ocean conditions. Understanding how the structure of coral skeletons will be impacted by multiple, co-occurring climate change stressors and how different coral species will respond is of paramount importance to understand the future of coral reefs. Here, we examined the skeletal structure of corals living in a mangrove system. Mangrove environments possess sub-optimal seawater conditions and provide a natural setting to examine the long-term effects of environmental stressors on the skeletons of resident corals as well as identify mechanisms that corals may adopt to survive. Using micro-computed tomography, we investigated the effects of environmental conditions ranging from mangrove habitat to open reef on the skeletal structure of Pocillopora acuta , Acropora cf. millepora and Montipora cf. digitata , in the northern Great Barrier Reef. Corals were collected from sites with differing distance from the mangrove system at Low Isles. Species-specific structural changes were recorded in coral skeletons in response to distance from the mangroves. P. acuta had more porous and thinner skeletons closer to the mangroves, where seawater conditions were sub-optimal. Montipora cf. digitata was able to maintain a robust skeleton in the mangrove environment. This coral also possessed more dispersed but larger corallites at these sites, potentially availing of increased food availability in mangroves as a mechanism to support skeletal growth. This study suggests the integrity of coral skeletons will be compromised by climate change stress but that some coral species may be able to utilise mechanisms to maintain normal skeletal growth under sub-optimal conditions.