Coral is from the phylum Cnidaria, which also includes jellyfish and sea anemones. They are radially symmetric, diploblastic animals and coral is of the basic polyp form meaning that it is a sessile animal that attaches the base of itís body to a surface, with itís mouth and tentacles facing upwards. The coral can secrete a skeleton of limestone (calcium carbonate) around its soft body and while each coral animal is small, they live together in large colonies all linked together as if they were one. Theses animals, working together, are master builders, producing giant structures such as the Great Barrier Reef. Although the coral form these huge structures, depending on the species, coral polyps may measure less than an inch to several inches in diameter. One of the largest corals, Fungia (mushroom coral), is a solitary coral that can extend 25cm in diameter, whereas colonial coral polyps are much smaller and average 1-3 mm in diameter. Coral colonies also vary in size, some corals only form small colonies, whereas others may form colonies several feet high. Star coral (Montastrea annularis) colonies reach an average height of 3-4 m.
Almost all types of coral polyps and some other cnidarians, contain algae (zooxanthellae) inside their epidermal cells. The algae and the polyp have a mutualistic relationship, the algae take in ammonia given off as a waste product by the polyp, and return amino acids. The coral also receives substances such as glucose and glycerol from the photosynthetic activities of the algae. This explains why coral reefs usually form near the surface of the water; so the algae have light to photosynthesise. The coral animals themselves feed on microscopic organisms and particles of organic debris.
Coral polyps have tentacles that are used for defense and for moving food to their mouth, depending on the subclass, a coral polypís tentacles are arranged in multiples of six or eight. The tentacles contain microscopic stinging threads called nematocysts. A nematocyst is a bulbous double walled structure containing a spirally folded, venom-filled thread with a minute barb at its tip. A tiny sensor projects outside the nematocyst, when it is stimulated physically or chemically, the capsule explodes and ejects the thread with considerable force and speed. The barb penetrates the victimís skin and injects a potent venom. Some of the soft corals secrete toxins to eliminate competitors, which are competing for living space on the reef. Some reef-building corals can actually digest the tissue of an invading coral.