Diver and a giant clam (Tridacna gigas). The clam's mantle tissues contain symbiotic singled- celled dinoflagellate algae (zooxanthellae), which provide it with nutrition and its colour. The clam spreads out its mantle tissue during the day so that the zooxanthellae receives the sunlight it needs to photosynthesise. Photographed off Maratua Island, in the Sulawesi Sea, Indonesia.