Coral:
Corals are solitary or colonial polypoid forms belonging to class Anthozoa and Hydrozoa of phylum Cnidaria. They secrete a skeleton of calcium carbonate forming cups and in each cup lies one polyp. The calcareous skeleton is secreted by a specifically modified ectoderm known as calicoblastic layer present at the basal region of the polyp.
Coral Reefs:
It proposed by Vaughan (1917). According to Vaughan, the coral reef is a ridge or mound of limestone, the upper limit of which is near the surface of the sea and which is formed of calcium carbonate by the action or organisms mostly the stony coral.
Types of Coral Reefs:
Coral reefs are of various types and forms but all of them could be resolved into three primary types namely –
Fringing Reef:
It is situated nearly 300 -500 meters away from the shore and is also called a shore reef. The most active zone of the Fringing reef is the sea-facing area where the corals grow very fast. This sea-facing area is commonly called the edge or front. Generally, navigable channels are absent between the shore and the reef. Sometimes a shallow water channel, 50 to 100 meters broad lies between the reef edge and the shore. During low tide, the water of the channel recedes and the bottom surface gets exposed, this area is called reef flat. Sometimes reef bed may be broken to form channels then this zone, the reef front as it is called is converted into a lagoon.
Example: Reefs of East Indies.
Barrier Reef:
They are more or less like fringing reefs but they have situated 1 km to 15 km away from the shore. A navigable lagoon separates the reef from the shore. The lagoons maybe 20-40 fathoms deep. It doesn’t follow the contour of the coast.
Example: The Great Barrier Reef of the north coast of Australia.
Atoll Reef:
It is more or less circular or horseshoe-shaped reef enclosing a lagoon that may be small or 90 miles across. This reef is broken by navigable channels small or large. The depth of the lagoon varies from 40-60 meters.
Example: Atoll Reef of Aldebra in the Indian Ocean.