Depth-dependent microskeletal features modify light harvesting in Turbinaria reniformis corals

Coral skeletal morphology plays a crucial role in modulating light
exposure in symbiotic algae, thereby influencing photosynthetic
performance and overall energy acquisition. This function is particularly
important in environments with limited light availability, such as in
mesophotic and deep-water reefs. However, quantifying light capture and
distribution within complex coral structures remains challenging. Using
optical coherence tomography and high-resolution X-ray scanning, we
explored the depth-dependent bio-optical properties of Turbinaria
reniformis from shallow and mesophotic environments in the Gulf of
Eilat/Aqaba, Red Sea. Our results reveal distinct skeletal layers: a
highly scattering superficial layer (~100 μm thick) and a deeper, more
light-penetrating layer. Mesophotic corals exhibited a higher scattering
coefficient and lower anisotropy of scatter, yielding increased
reflectivity, thereby optimizing light use under low-light conditions.
Structural features played distinct optical roles: coenosteum grooves
facilitated forward scattering and light trapping, while protruding
features such as spines and septa increased surface reflectivity and
distributed light more broadly. Light simulations further demonstrated an
enhanced fluence rate at the skeleton-water interface, with mesophotic
corals amplifying the available light up to 2.7-fold. These findings
reveal previously unrecognized depth-dependent adaptations that enhance
coral light-harvesting efficiency, providing insights into how skeletal
morphology supports coral survival across varying light environments.