- Hard corals are made of rigid calcium carbonate (limestone) and appear very much like rocks. Each polyp secretes a hard exoskeleton made up of calcium carbonate. Hard coral polyps have a chalky internal skeleton that stays in place after they die. Beneath each polyp is more coral, made up of dead exoskeletons. As each generation of polyps dies, the coral grows a bit larger, and because each polyp is so small, hard corals grow at a very slow rate. They do not move, but they harbor various types of algae that give them their various colors. Sometimes, huge colonies of hard corals live together and grow into huge masses, such as the Great Coral Reef off the northeast coast of Australia, the world's largest coral reef.
- Soft corals are also composed of some rigid calcium carbonate, but it is blended with protein so it is less rigid than hard corals. These corals are "rooted," but because they have no exoskeletons, they sway back and forth with the currents, appearing to be more like plants blowing in the breeze.
- Hard corals, scientifically known as "scleractinians," can often be viewed in public aquariums. Brain coral is one type of hard coral, and the many convolutions in its surface account for its name. Elkhorn coral and pillar coral are two more examples of hard corals.
- Soft corals, also called "gorgonians," are also used to add interest to aquariums. The polyps have no internal skeleton, so the coral retains its suppleness. Some examples of soft corals are sea fans and sea rods that bob and bow with the movement of the water.
- For corals to grow properly, they have a few requirements. They need shallow, clean water, sunlight and water that is between 17 and 28 degrees Celsius. Also, they need a supply of food, and get oxygen and carbohydrates from the algae that grow on the coral. In addition, they eat very tiny types of algae, zooplankton and phytoplankton, that float up from the bottom of the sea during the night. Through their microvilli, the corals take in dissolved organic molecules, including sugar and protein from dead animals and fish feces.
- Life begins for all coral polyps as a swimming, head-of-a-pin-size polyp that alights on a hard surface, never to move from it again. To form "babies," the polyp will either "bud" (form "sprouts" of new polyps from its sides) or release eggs and sperm that unite to form new polyps.
