Hard coral has a hard calcium carbonate structure and in general is very slow growing because it takes time to secrete this hard skeleton.
Hard coral is considered by scientists to be animal, mineral and plant. The skeleton structure is the mineral part, polyps the animal part and the symbiotic zooxanthellae that live in the coral are the plant part.
Coral Polyps are a gelatinous anemone like structure that take nutrients from the water and combine this with energy provided from photosynthesis conducted by the zooxanthelle to produce the calcium carbonate skeleton. The polyps on hard coral are generally difficult to spot during the day as they tend to feed at night.
Most corals grow between depths of 10 to 30m because of the requirement for sunlight to complete photosysthesis. Most species of coral are colonial and consist of thousands of polyps interconnected by living tissue. A colony's structure is influenced by species but also by the intensity of currents, water quality, depth and the action of the waves. For example brain like structures channel and slow down the passing water funneling it toward the polyps. Delicately branched coral prefer to grow in deeper waters with gentle currents.