大象传媒

David Attenborough goes scuba diving at night on the Great Barrier Reef and is surrounded by corals. Coral look like plants, but are in fact animals. At night their tiny, flower-like polyps come out to feed. But it is in daylight that the secrets of the corals are revealed - they cannot survive without sunlight, which explains their plant-like shapes, as they grow towards the light. For within their bodies there are microscopic algae which make starch and sugar. Microscope shots show the masses of tiny algae inside a coral polyp. The corals have partly digested the cell walls of the algae, so they can feed on the starch and sugar the algae produce. In return the algae get nitrogen and phosphates from the coral, as well as protection inside a stony skeleton.