Reduced skeletal density and mechanical resistance in mangrove-associated Pocillopora sp. corals
Coral skeletal integrity underpins reef habitat complexity and coastal protection, yet it remains unclear whether corals persisting in chemically variable marginal habitats, such as mangrove-associated reefs, maintain sufficient mechanical resistance to withstand physical disturbance. We combined micro-computed tomography (micro-CT) morphometric analysis with compression testing to quantify skeletal microarchitecture and compressive mechanical performance of Pocillopora sp. skeletons collected across a habitat gradient at Low Isles, Great Barrier Reef, from mangrove-associated sites to exposed reef sites. Baseline skeletal properties varied among sites, with the Proximal Reef site having the densest and least porous skeletons, while mangrove-associated sites had the most porous skeletons. Similarly, Proximal Reef skeletons exhibited two-to-three-fold greater compressive stiffness and peak force than mangrove-associated skeletons. Percentage object volume was a stronger predictor of compressive stiffness than surface area-to-volume (S/V) ratio, indicating that bulk skeletal density better captured load-bearing capacity than surface architectural complexity. Structural failure mode also differed markedly among habitats: mangrove-associated skeletons underwent catastrophic fragmentation of their open, porous framework, whereas Distal and Proximal Reef skeletons failed through discrete crack-plane propagation while largely maintaining overall structural coherence. These results indicate that the physiological stress tolerance associated with mangrove habitats occurs alongside reduced mechanical resistance and more destructive failure modes, suggesting a potential trade-off between chemical stress tolerance and structural investment. Assessments of resilience in marginal coral populations should therefore consider skeletal mechanical performance alongside physiological tolerance, particularly as climate change intensifies storms and alters hydrodynamic disturbance regimes.