The giant clam photosymbiosis is a physically optimized photoconversion system for the most intense sunlight on Earth

Giant clams are photosymbiotic with unicellular algae (\"zooxanthellae\") organized in the clam's mantle tissue. This tissue has an especially low albedo for a photosynthetic system, generally less than 10\% at all visible wavelengths. This efficient absorbance of light occurs in the ecological context of the high solar irradiances in intertidal habitats near the equator. At these light levels, photosynthetic systems typically adapt to absorb less light in order to prevent radiative damage to chloroplasts. Giant clams are therefore unusual.  If the giant clam photosymbiosis proves to be simultaneously efficient at absorption and at phototransduction at these irradiances, they are potentially remarkably productive and an important source of bioinspiration. We showed previously that the clams organize algae into vertical pillars in the mantle tissue. The clams' iridocytes, or optically structured skin cells on the surface of the tissue, then function to evenly distribute incoming solar irra...