Habitats: Coral Reefs - Characteristics

Coral reefs are found almost exclusively in the seas and oceans between the Tropic of Capricorn and the Tropic of Cancer. In this region, water temperatures are warm and stable year-round (64 - 86 degrees Fahrenheit, 18-30 degrees Celsius), and longer days bathe the waters with sunlight.

Though thriving coral reefs are a collection of many different plant and animal communities, the members of individual coral colonies (polyps) actually build the reef's limestone, or calcium carbonate, structure. Polyps consist of a tube and an oral disc, or mouth, surrounded by tentacles, which the polyps use to capture food. The tube and oral disc sit inside a calcium carbonate cup. As polyps grow, they produce small buds.

Each type of polyp buds in a different way, leading to a large variety of shapes and sizes of coral colonies ranging from the rippled ball of brain coral to elegant fans, flat discs, graceful branches and columns.

As a single polyp dies, its soft tissue decays, but the calcium carbonate cup remains. Other polyps build on top of the cup, and when they die, other polyps will build on their cups. Over time, this process creates larger and larger coral reefs. Several kinds of algae help hold the reef together by growing between the colonies of coral polyps and keeping the sand that accumulates there from washing away.

Living within each coral polyp is a small plant, a single-celled algae called zooxanthellae. The algae provide food to the polyps by photosynthesis, which means the algae use sunlight to break carbon dioxide down into oxygen and carbohydrate. In turn, the polyp provides food to the algae with its waste products. The algae store the waste as ammonia and break it down into nitrogen and phosphorus, which the algae use for energy. This beneficial relationship is a type of symbiosis called mutualism. Both the polyps and the algae are helped and neither is harmed by their relationship. Polyps can also draw food directly from the water, using their tentacles to catch drifting plankton.