- Coral-algae symbiosis is a key feature of tropical corals and is highly dependent on the efficiency with which solar energy is attenuated by the coral. Scleractinian corals are among the most efficient light collectors in nature by modulation of the internal light field in the coral skeleton. Interestingly, coral skeleton particles composing the sandy bottoms in reef margins sustain these optical characteristics. In the present study we examined two free-living coral species Heterocyathus aequicostatus (Caryophyllidae) and Heteropsammia cochlea (Dendrophylliidae) common on biogenic coarse carbonate sand of the Great Barrier Reef (GBR) but absent from fine sand at the same depth. In coarse carbonate sand light penetrates a few millimeters below the surface and propagates along horizontal distances of a few centimeters. In fine sand, almost all of the light is reflected back to the water column. For photosynthetic sand-dwelling organisms such as the studied species, with over one third of their surface area facing the substrate, light flux to their underside may be beneficial. A correlation was found between the diameter of these corals and the distance that light may travel in the sand under the coral. Laboratory and field measurements prove that the symbiotic algae at the underside of the corallites are photosynthetically active even when the coral is partially buried, implying sufficient light penetration. Other organisms in the study site such as fungid corals and forameniferans, with different morphologies, have different light trapping strategies but are also photosynthesizing in their underside. The importance of the substrate type to the performance of the three main partners of the symbiosis in H. aequicostatus and H. cochlea (coral, endosymbiotic algae and a sipunculan worm) is highlighted striking example of co-evolution of these organisms.