Algae
Other types of Cnideria, such as sea anemones and jellyfish, also contain algae. Jellyfish with algae behave so that their partners get the best light during the day, and descend to depths at night, where the water is rich in nitrates and brown with decay. Sea slugs and clams are also well known for harbouring algae. Both groups are molluscs. The sea slugs graze on coral, and are the same colour as the coral they graze. They are able to separate the algae from the polyp tissues they digest. The algal cells are moved to its tentacles, where they continue to live. The otherwise defenceless slug gains both camouflage and nutrition.p204 The giant clam keeps algae in its mantle, which is revealed when the clam is open. The coloured mantle has places where the skin is transparent, and acts like a lens to concentrate light on the algae beneath. When the algae get too numerous, the clam digests them.p203