|Pink Noble coral
|Orange Noble coral
|Large cup coral
|Small cup coral
|Hard corals are most often referred to as corals that contain a 'hard' calcium skeleton. Their structure consists of a calcium framework, or base, of which a polyp or polyps extend off of. These polyps are generally large ( for the sake of this web page they are ), fleshy and consist of one or more individuals. The fleshy, or bulbous polyps are used for capturing sunlight, as well as, food particles.
A large number of 'hard' corals contain within the fleshy polyp's) a symbiotic algae know as Zooxanthellae. This algae lives within the fleshy tissue and utilizes sunlight via a process called photosynthesis to create a food source for the coral. One of the by-products of the photosynthetic process is that the polyp's) will secrete calcium at their base. It's this depositing of calcium that forms the 'hard' skeleton structure.
In addition to sunlight the coral will also feed on small particles of living matter, dissolved organics and debris's. Most of these 'hard' corals have tentacles that are used for capturing these particles. Some produce the tentacles when they sense food in the water, others only produce them at night. The tentacle is armed with a nematocyst, or stinging cell, that is used to sting its prey or to deture encroaching, competitive, corals.
Because of these corals light and food requirements one needs to provide lighting that is equal to the suns spectrum and intensity taking into consideration the depth and turbidity at which the coral normally resides, water current parameters, and its proximity to its neighboring corals.