Corals are colonial animals made up of many small individuals called polyps. Each polyp resembles a jellyfish turned upside down, and sites in it’s own cup, called a calice or coralite. On hard corals the polyps are always visible to the naked eye. The top of the polyp consists of a mouth surrounded by tentacles, that are covered in stinging cells used to capture prey. Mostly corals extend their tentacles at night to feed on zooplankton, which when captured, is pulled into the mouth of the corals. The coral does not have a digestive system, but just a digestive gut, so it will feed, digest, then excrete every 24 hours or so. The skin of the coral is clear, but inside the tissue of the coral is full of unicellular algae called zooxanthallae. This unicellular algae are actually a plant, but live inside the coral to take advantage of the constant light levels and supply of nutrients inside the coral body. In return, the zooxanthallae provide energy to the coral in the form of sugars and carbohydrates which they create using photosynthesis.
The coral animal, along with its algae, grows on a skeleton of coral created from calcium in the sea water. A single coral species can grow in a wide variety of shapes and sizes depending on the environmental conditions (light, currents, sedimentation, etc). Assessing the shape of the coral gives us information on the conditions present on the reef. Below you will find a commonly accepted list of coral growth forms, which describes the shape of the overall colony.