Supplementary and dataset for: Species-specific vulnerability of northern Red Sea mesophotic corals to accelerated warming

Mesophotic coral ecosystems have gained attention for their potential role as refugia from climate-driven coral bleaching, yet their resilience to prolonged marine heatwaves remains uncertain. Here, we experimentally assessed the thermal tolerance of two Red Sea corals with contrasting depth distributions: the mesophotic specialist Leptoseris glabra and the depth-generalist Acropora squarrosa. Through controlled thermal stress experiments simulating strong to extreme marine heatwave events, we quantified photophysiological, optical, and biochemical responses. Under thermal stress, L. glabra exhibited severe declines in photosynthetic efficiency and energy reserves, coupled by a three-fold increase in tissue reflectance, indicative of extensive symbiont loss. In contrast, mesophotic A. squarrosa maintained stable photophysiology and energy reserves, whereas its shallow conspecific showed marked declines in photosynthetic efficiency, symbiont density, and lipid stores. Skeletal optical analysis revealed that shallow A. squarrosa exhibited higher scattering coefficient and reflectance than its mesophotic counterpart, while L. glabra shared similar optical properties with mesophotic A. squarrosa. These findings suggest that enhanced skeletal light scattering may have exacerbated bleaching in shallow A. squarrosa, whereas reduced light exposure and greater lipid content contributed to mesophotic A. squarrosa’s resilience. Critically, 20-year in situ temperature analysis revealed accelerated warming across intermediate to upper mesophotic depths compared to shallow reefs. This dual threat of species-specific vulnerability combined with accelerated warming at presumed refuge depths, suggests that refugia capacity in mesophotic ecosystems may not be universal across all taxa, but instead depend on species-specific traits.