There are two types of coral; hard coral and soft coral. Hard corals, like brain coral and staghorn coral, are the reef makers. Soft corals, like broccoli corals and mushroom coral, do not contribute to reef building.
Coral reef refers to the limstone structure produced when coral polyps excrete an exoskeleton of calcium carbonate. Even though this exoskeleton grows at a very slow rate, typically 1-2cm annually, it accumulates over hundreds of years to form massive reef networks. These are the largest structures on earth of biological origin.
Worldwide distribution of coral
Coral reefs occure in both temperate and tropical waters. The tropical reefs are much more prolific with 92% of the world's reefs exisiting in the Indo-Pacific region Atlantic and Caribbean coral reefs are much less significant accounting for only 7.6% of the worlds total reef coverage.
Reefs are rare on the west coasts of America and Africa as strong cold coastal currents prohibits coral growth. They are also rare around the north eastern fringe of South America due to the freshwater release from the Amazon basin.
Coral polyps structure
All coral polyps share two basic structural features. The first is that they all have a gastrovascular cavity and mouth. Food is consumed and some waste prodcuts are expelled through this mouth. The second feature is that all corals contain a circle of tentaclees at the hood of the mouth. The tentacles help the coral to capture and ingest plankton for food and also to clear away debris from the mouth.
Better living through symbiosis
Most corals live in a symbiotic relationship with an algae called zooxanthellae. The coral provides the algae with a protected environment and the compounds necessary for photosynthesis. These include carbon dioxide, produced by coral respiration, and inorganic nutrients such as nitrates, and phosphates, which are metabolic waste products of the coral.
In return, the algae produce oxygen and help the coral to remove wastes. Most importantly, they supply the coral with organic products of photosynthesis. These compounds, including glucose and amino acids, are utilized by the coral in the manufacture of proteins, fats, and carbohydrates, as well as the synthesis of calcium carbonate (CaCO3).
The large reliance on zooxanthellae photosynthesis explains why the tropical shallow water corals only exist above 50m depth and thrive in a temperature of 26°C - 27°C.