The symbiotic coral Leptoseris fragilis lives in the Red Sea at depths of 95–145 m. Symbiotic dinoflagellates (zooxanthellae) themselves possess well known adaptations to low light intensities. In L. fragilis we found indications that light amplifying mechanisms of the host improve photosynthesis of the symbionts. Light of short wavelengths is absorbed by host pigments which transform short into longer wavelengths. The transformed light is more efficient for photosynthesis. Action spectra measurements of photosynthesis demonstrated the amplification of photosynthetically active radiation. Monochromatic light of 387 nm (outside the main absorption maxima of the algal pigments) at subsaturation photon flux densities was as effective photosynthetically as polychromatic light of 415–490 nm, which fits the absorption maxima of the zooxanthellae.