The Coral Reef Ecosystem

The actual coral reef structure is created by certain species of corals. These corals, appropriately known as reef-building (hermatypic) corals, build the reef framework as they produce their calcium carbonate skeletons. The coral polyps, which usually live colonially, remove calcium carbonate from the seawater and use it to build the skeletons that protect and support their tiny, cup-shaped bodies. The reef structure expands as new generations of polyps build their skeletons on the skeletons of older generations. This build-up occurs slowly over a long period of time.

Besides corals, there are many, many other species of organisms associated with coral reefs. For these organisms, the coral reef structure provides an ideal habitat in which to find food and shelter. Algae on the reef remove excess nutrients and add needed oxygen as they photosynthesize.

Some species, known as coralline algae, are calcareous and act like cement to help hold the reef structure together. Other species, known as zooxanthellae, live symbiotically with corals in their tissues.

Algae, in turn, is food for animals who are grazers or herbivores, such as conchs, sea urchins, and parrotfish. These organisms are then preyed upon by other animals, such as moray eels, barracuda, and sea turtles. Some consumers are specialized, meaning they only eat a certain kind of prey, while others are generalists and feed on a wide variety of prey items. Some of the organisms on a coral reef are nocturnal, hiding in cracks and crevices during the day.

Because reef-building corals and their symbiotic zooxanthellae require special conditions in order to flourish, coral reefs are only found in certain areas. They flourish in warm (70-85øF), salty water that is very clear. They are found at depths less than 200 feet, in order for the corals' zooxanthellae to receive enough light for photosynthesis. The area must also have enough water movement to bring plankton and oxygen to the reef and its inhabitants.

The coral reef ecosystem is a very stable one. Most coral reefs do not experience tides, and those that do do not have the extreme tidal fluctuations seen in temperate ecosystems. There is only a slight seasonal change in the average water temperature and amount and intensity of daylight. This extremely stable environment is one of the reasons tropical coral reefs are able to support such a tremendous diversity of flora and fauna.

Often times, lagoons are located between the reef and shore. Lagoons have calm waters and sandy bottoms and are ideal environments for seagrass beds. Dense blades serve as protective nurseries for many reef species, while roots and underground stems stabilize the soft bottom. Seagrasses and the algae that grow on their blades are important producers of food and oxygen for the entire reef ecosystem.