Although many corals resemble plants, they are actually members of the animal phylum Cnidaria. Most corals are colonial, which means that each coral is made up of many individual polyps connected by living tissue (the coenosarc). Each polyp has a cup-like shape with a ring of tentacles around a central opening (pharynx) that functions as both mouth and anus. The tentacles are tipped with stinging cells called nematocysts. Corals use the nematocysts to defend themselves and to capture prey. The body wall consists of three cell layers: the outer or ectoderm, the middle or mesoderm, and the inner or endoderm. There is no skeleton inside the polyp itself. Instead, the polyps sit on top of an external skeleton that is made from the polyp's secretions.
Corals can be divided into two main types, the hard corals (stony corals, or scleractinians) and the soft corals (gorgonians or octocorals). As their names might suggest, these two types of corals have very different skeletal structures, but there are other differences too. The soft corals have a flexible skeleton, made of a protein called gorgonin. Their skeleton also contains calcium carbonate, but only in small clumps called spicules. The polyps of soft corals have eight tentacles (hence the name octocoral since the word octo means eight). There are 17 species of soft corals in Bermuda, including sea whips, sea fans, and sea feathers.
The stony corals are the major reef-building species because of their rigid calcium carbonate skeletons. Besides their skeleton, the stony corals are also distinguished by their tentacles, which occur in multiples of six. There are 21 species of stony corals in Bermuda, as compared to 70 Caribbean species and 400 Indo-Pacific species, probably because of the cold water temperature in Bermuda compared to the other regions. Hard corals have three types of morphologies, or growth forms: massive forms such as brain corals, encrusting forms like the star corals, and branching forms such as Madracis. The different growth forms represent adaptations to different environmental conditions. The massive and encrusting forms are wave-resistant. Branching forms are less wave-resistant, but they can survive higher sedimentation rates than many massive corals. These influences are also discussed on the reef formation page.
Corals use their tentacles to capture zooplankton (small animals that live in the water.) Most corals only extend their polyps and tentacles at night when zooplankton is most abundant, but some corals (especially soft corals) keep their polyps open throughout the day. Many corals have single-celled algae (called zooxanthellae) that live within the coral's innermost tissue layer. Both the corals and the zooxanthellae benefit from the arrangment. The algae uses the sun's energy to convert carbon dioxide from the seawater into energy-rich sugars and fats. Some of this "junk food" is given to the coral animal and helps the coral grow and produce its skeleton faster than a coral without the zooxanthellae. The zooxanthellae also gives the coral its color. In return, the algae have a safe place to live within the coral tissue and the algae uses the coral's waste nutrients for growth. This type of arrangement -- where both organisms live together and benefit from the relationship -- is called symbiosis and when both organisms benefit, it is a mutualistic symbiosis.
To learn more about how corals reproduce, click here to jump to the page on the life cycle of coral..