Species-Specific Bleaching Dynamics and Zooxanthellae Responses of Acropora and Montipora Corals Under Ex-Situ Thermal Stress

Global marine heatwaves pose a severe threat to coral reef ecosystems, triggering mass bleaching and the breakdown of coral–dinoflagellate symbiosis. This study evaluated the bleaching percentage and Symbiodiniaceae (zooxanthellae) density dynamics of three scleractinian corals—Acropora millepora, Acropora tenuis, and Montipora capricornis—under controlled ex-situ thermal stress. A total of 12 fragments per species per treatment (n = 4 for bleaching assessment; n = 12 for zooxanthellae density), obtained from genetically distinct parent colonies, were distributed across independent closed-system aquaria. An ex-situ laboratory experiment was conducted over 28 days, comparing a control group (28 °C) with a thermal stress treatment (ramp-up to 32 °C). Bleaching area was quantified by standardized top-down photographs, with necrotic tissue excluded from analysis; zooxanthellae densities were quantified using a Neubauer haemocytometer and normalized to coral surface area (cells/cm²). A Linear Mixed Model (LMM), with individual fragment identity as a random intercept, was used to analyze repeated-measures data. The results revealed a highly significant interaction between coral taxa and exposure duration (p < 0.001). Under 32 °C, Acropora tenuis demonstrated the highest thermal sensitivity with bleaching rates reaching 84.6% ± 8.9 by Day 7, escalating to 99.1% ± 0.85 by Day 14, while Acropora millepora reached 100% bleaching by Day 14. Conversely, Montipora capricornis demonstrated higher initial thermal resilience, sustaining a bleaching rate of 66.0% ± 5.2 at peak thermal stress (Day 14), before exhibiting a severe delayed bleaching response (99.6% ± 0.35) during the recovery phase (Day 21). The delayed response in Montipora capricornis may reflect greater tissue thickness and symbiont retention capacity, though lipid reserves and other physiological parameters were not directly measured. These findings indicate that branching Acropora species are highly sensitive ecological indicators susceptible to prolonged thermal stress, whereas foliose Montipora exhibits partial short-term resilience before ultimately succumbing to severe delayed bleaching. These results highlight species-specific vulnerability of coral morphologies to projected climate change scenarios and underscore the need for cellular-level monitoring to complement visual reef-health assessments.