Coral reefs are sea water structures made of calcium carbonate (CaCO3) secreted by living organisms. Thousands of organisms thrive in coral reef ecosystems; however, only a small fraction manufactures the limestone required to build coral reefs. Stony (hermatypic) corals are one type of corals responsible for building coral reefs. Stony corals live in a symbiotic relationship with a one celled algae known as zooxanthellae. Although, coral reefs comprise a small percentage (Less than 1 %) of the world’s oceans, they provide the habitat of roughly one quarter of the Earth’s marine species.
Symbiosis is a term derived from the words “sym,” meaning together and “biosis,” referring to life. Symbiosis occurs when two organisms interact in an association that can benefit one or both organisms. There are different kinds of symbiotic associations, such as mutualism, comensalism and parasitism. Mutualism association is the most common of the three and is the symbiotic asociation in which coral reef-building organisms interact with their symbiont algae (zooxanthellae).
Coral polyps live in a mutualistic association with a photosynthetic algae. several million single- celled zooxanthellae produce pigments that give the coral reef their characteristic varied colors. Inside the sac of each coral polyp lives zooxanthelae. The algea provides oxygen and the energy required for the polyp to live. The polyp, in return, supplies the algea with carbon dioxide and other nutrients needed for the algae to survive. The algea also gets benefited from the coral’s protective calcium carbonate skeleton. Coral polyps usually thrive on surface waters, permitting the algea to use the sunlight for photosynthesis. The mutualistic relationship promotes the capacity of reef-building corals to secrete limestone.
The photosynthetic pigments in corals are provided by their symbiont zooxanthellae, but when corals are under stress, they typically expel their symbiont algae and the concentration of photosynthetic pigments decreases. This makes the algae to become transparent, and the coral white limestone skeleton is shown through the transparent coral’s tissue. This is a type of disease known as coral bleaching. Coral bleaching has been attributed to increase ocean water temperatures due to global warming. Bleaching in some coral species have occurred due to bacterial infection, as well. When coral bleaching is not too severe, the stressful conditions may decrease, and the affected colony of polyps may regain their symbiont algae.
In the coral reef marine ecosystem, diverse animal species rely on each other for their survival. There are many other symbiotic associations between coral reef animals and plants. Some small fish feed on parasites attached to the bodies of other fish. Certain fish live among the jelly fish tentacles which allow them to trap other fish and share the meal with the jelly fish. The carrier crab picks up sponges and seaweed, allowing the crab to have protection against predators while the sponges and seaweed attain a better food selection. According to coralreef.noaa.gov, coral reefs suffering severe damage after coral bleaching can take many years, even decades to recover.