Coral polyps depend on zooxanthellae for oxygen and carbohydrates and zooxanthellae depend on polyps for carbon dioxide and shelter. Another intriguing example of mutual symbiosis is with certain shrimps and small fish called Cleaners. Cleaners feed on the parasites and damaged tissues of the skin and mouth of larger, host fish. Within a coral reef there is intense competition among species for food and space. By developing such complex patterns of behavior it has enabled these wonderful species to coexist.
Coral reproduction varies depending on the species.
Some species such as brain coral or star coral are hermaphrodites , producing both sperm and eggs at the same time. Other coral species such as Elkhorn coral or boulder coral produce single-sexed colonies. Thus, all of the polyps in one colony produce only sperm and all of the polyps in another colony produce only eggs. Corals reproduce both sexually and asexually . In most species, the egg and sperm are released into the water where random fertilization occurs, resulting in a baby planula larva .