Corals are tiny animals related to jellyfish. A coral colony is composed of many individual coral animals, or polyps, that may grow or divide and create new coral polyps. These colonies work together to protect their living space and share nourishment.
Corals have stinging cells called nematocysts that they use to capture prey and defend themselves against predators. Corals also have a symbiotic relationship with zooxanthellae, a type of algae that lives in their tissue and provides nourishment through photosynthesis. During the day, most coral colonies retract their tentacles and obtain as much as 90 percent of their nourishment from their zooxanthellae.
Corals can reproduce either asexually or sexually. Asexual reproduction is most common and occurs when a portion of living tissue or fragment of a colony is broken away from the parent colony. Sexual reproduction occurs when an egg and sperm combine to form larvae, which then settle onto suitable substrate to become a new coral colony.
Corals may be either hard (stony) or soft. Both types of corals are essential members of a healthy coral reef ecosystem. Hard corals are composed of soft tissue, but they secrete calcium carbonate, or limestone, to create a stony-like, cup-shaped skeleton around themselves. This skeleton provides the primary physical structure of a coral reef. Marine plants and animals, including other corals, grow on the skeletons of dead hard corals, and over thousands of years a reef forms. These reef-building hard corals grow less than 1 centimeter per year. Their age can be determined by examining their growth rings, much like counting rings in the trunk of a tree. However, some hard corals, such as elkhorn and staghorn, grow as much as 10 centimeters per year, but their branches are easily broken in strong storms or as a result of human impact.
Like hard corals, soft corals are composed of colonies of soft-bodied polyps with stinging tentacles capable of capturing prey. They also have symbiotic zooxanthellae living in their tissue. However, most soft corals do not secrete calcium carbonate skeletons. They do have some hard cells within their tissues that provide rigidity, and some produce holdfast structures or long, rod-like internal supports. In addition, the tentacles of soft coral polyps have tiny feather-like projections, giving them a bushy, plant-like appearance. The extensive branching of several soft coral colonies enhances this plant-like appearance. Soft corals generally grow much quicker than their hard coral relatives.
