Coral reefs are one of the most diverse ecosystems on our planet, rivalling that of the tropical rainforest. They have been built over thousands of years (some of the oldest reefs began growing about 25 million years ago) by tiny calcium-producing organisms, and are a haven for thousands of life forms. The coral reefs are unique in that living examples from nearly every group of organisms, representing a billion years of evolution, can be found there. In spite of this, we have spent less time exploring the world’s oceans than we have the surface of the moon.
Corals are classified as Cnidaria. They share the class Anthozoa with sea anemones, sea pens and sea pansies. Of the 6,000 known anthozoan species, 2,500 are corals. Corals are found in all oceans of the world, from the tropics to polar regions, to depths of 6,000m (19,700ft).
Reef-building corals are generally found at depths of less than 46m (150ft), where sunlight penetrates. These corals have a symbiotic relationship with algae and need sunlight to grow and thrive. Clear water, where light reaches the algae that promote polyp calcification, speeds growth. Reef-building corals require warm ocean temperatures of 20-28 degrees centigrade.
Warm water flows along the Eastern shores of major land masses and reef development is generally more abundant in areas that are subject to strong wave action. Waves carry nutrients and oxygen to the reef as well as distributing coral larvae and preventing harmful sediments from settling on the reef. Precipitation of calcium from the water is necessary to form a coral polyp’s skeleton. This precipitation occurs when water temperature and salinity are high and carbon dioxide concentrations are low, conditions typical of shallow, warm tropical waters.
Types of coral reef
Fringing reefs – which border the coast closely
Barrier reefs – which are separated from the land by a lagoon
Atolls – which develop at or near the surface of the sea when islands that are surrounded by reefs subside.
