Symbiosis simply put is a life together, and for ocean forests of the coral reef's symbiosis is a partnership between coral and algae forming the basis of all symbiotic life in the reef community.
At the base of a reef is the living coral animal existing in a co-beneficial relationship with single cell algae known as zooxanthellae which lives inside the tissues of the coral polyps, taking in carbon dioxide and through photosynthesis emiting oxygen to the living coral polyps providing life and colour. Absorbing calcium in the sea the limestone column shaped creatures build themselves up through depositing a calcium carbonate, limestone, around their base as they grow. Attaching to each other and forming a colony the coral polyps live and when they die off their limestone skeletons continue to support the growing community as a reef is forming. The intricate coral structure shapes over time trapping rock, sand and grasses and creates a home to a sustainable bio-diverse system and structured food-chain within the reef.
Many other symbiotic relationships exist within a coral reef. Plants and animals create co-habitant relationship and live together with mutual benefit. For the coral themselves mutual relationships include the Herbivorous fishes which have jaws and teeth adapted to scraping algae off the corals and excreting sand back into reef, and as well some shrimp and crabs live on the coral surfaces cleaning away the dead surfaces and parasites.
Equally in reef life many parasites which could cause disease and ill health to their host are picked off in a meal by other fish. Jellyfish stings can be harmful to most fish but some have adapted an immunity allowing them to hide within the tentacles and sharing the co-trapped catch. Sponges and seaweeds makes for great camouflage on the back of a decorator crab as does the defence offered to a carrier crab by carrying a sea urchin on its back, both together attracting more food between them to share than apart. Some reef animals live in the shelters or burrows created by other hosts feeding of scraps and even waste of the host.
The Mutualism partnerships are one form of symbiosis, with Parasitism being another form occuring when one is benefiting at the expense of another. Commensalism being the third form of symbiosis when one side benefits and the other is indifferent to the relationship of give and take. In a coral reef food chain almost all creatures live in some form of symbiosis in the delicate balance of life on the reef.