Microscale light management and inherent optical properties of intact corals studied with optical coherence tomography

Coral reefs are highly productive photosynthetic systems and coral optics studies suggest that such high efficiency is due to optimized light scattering by coral tissue and skeleton. Here, we characterize the inherent optical properties, i.e. the scattering coefficient, ms, and the anisotropy of scattering, g, of eight intact coral species using optical coherence tomography (OCT). Specifically, we describe light scattering by coral skeletons, coenoarc tissues, polyp tentacles and areas covered by fluorescent pigments (FP). Our results reveal that light scattering between coral species ranges from m _s ¼ 3 mm ²¹ (Stylophora pistillata) to m _s ¼ 25 mm ²¹ (Echinopora lamelosa). For Platygyra pini, m _s was 10-fold higher for tissue versus skeleton, while in other corals (e.g. Hydnophora pilosa) no difference was found between tissue and skeletal scattering. Tissue scattering was threefold enhanced in coenosarc tissues (m _s ¼ 24.6 mm ²¹ ) versus polyp tentacles (m _s ¼ 8.3 mm ²¹ ) in Turbinaria reniformis. FP scattering was almost isotropic when FP were organized in granule chromatophores (g ¼ 0.34) but was forward directed when FP were distributed diffusely in the tissue (g ¼ 0.96). Our study provides detailed measurements of coral scattering and establishes a rapid approach for characterizing optical properties of photosynthetic soft tissues via OCT in vivo.