Corals are association of a group of tiny organisms called coral polyps which belong to the class anthozoa of the phylum cnidaria. Each coral polyps are individual organisms which has a sac-like body and an opening (mouth) encircled by stinging tentacles. Corals are mainly classified into two based on whether they secrete calcium skeleton or not. Corals which do not form skeleton instead contain structures within their tissues called spiracles that support their bodies are known as soft coral, whereas the corals which secretes calcium-based skeleton for their support and protection are called hard corals.
Coral reefs are hard skeletal structures formed by the hard corals for their growth and protection by extracting abundant amount of calcium carbonate from the surrounding sea water. The skeletal cup formed by each coral polyp is known as corallite. Corallite consist of an inner surface calyx with vertical blades inside it called septa. In some corals the blade like structure grows outside the corallite wall known as costae and if the corallite wall is absent it is known as septo costae. The septa, costae and septocostae may be thick, thin or variable in size and have ornamentation in the form of teeth (Dr. Pamela hallock, 2009) Coral growth is a slow process some grow about 2-20mm per year as the corals grow and die, they left behind the skeleton for other corals to grow. They are usually seen where the sea is shallow (less than 100m) and have a temperature of about 25°-29°C.corals require sunlight for their growth.
Type of coral reefs
The corals are differentiated in large scale reef morphology based on; the size and shape of a reef, and its relation to nearby land [William et. al;2018]. Based on that the main four groups of corals are Fringing reefs, barrier reefs, atolls and platform reefs.
- Fringing reefs or shore reefs are mainly seen close to the border of the coastline or separated by a narrow stretch of water. These types of reefs are mainly seen in the red sea and Caribbean. These types of fringes have simple structure which can be divided into three zones as: fore reef, reef crest, and back reef [Castro P; 2008].
- Barrier reefs :This type of coral reefs is parallel to coastal area but are separated by deeper and wider lagoons. They are usually seen about 10-100km from the shore. The Great Barrier Reef in the Australia is the largest barrier reef in the world [Castro P; 2008].
- Atolls are ring shaped coral reefs surrounded by water body’s called lagoons. They are mostly seen in Pacific Ocean and Indian Ocean. Atolls are remnants of fringing reefs [Jessika caril;2016]. According to Darwin’s theory the growth of coral atolls takes place as the reefs stretches towards sunlight while the oceanic island sinks under the sea. For the atoll to persist, continued erosion or subsidence must be at a rate slow enough to permit reef growth upward and outward to replace the lost height.
- Platform reefs are usually seen in sheltered seas and far offshore. They are flat-topped reefs With small and very shallow lagoons. This type of corals has a primary radial growth pattern and is mostly seen in continental shelves. The shape of these corals is mainly determined by direction of force of water currents and the shape of the base on which they grow [Castro P; 2008]
