1. CORAL REEFS

2. Corals or polyps as living organisms
It is significant submarine features. formed due to accumulation and compaction of skeletons of lime secreting organisms known as coral polyps.
Found in the tropical oceans confined between 25°n-25°s latitudes and live on lime.
The coral reefs have the following components:
Corals or polyps as living organisms
Corallites, the external skeleton or house of the corals, and polyps
Reef, cemented and hardened structure of calcium carbonate.

3. Conditions for the growth of coral
Temperature of seawater- corals are temperature sensitive. They cannot thrive in either cold or very warm sea temperature. Corals can grow in a temperature range of 20°c .
Depth of sea water- corals grow successfully in the ocean water up to the depth of 30 meters below sea level but they cannot survive in seawater deeper than 80 meters.
Turbidity of sea water- high turbidity of seawater i.e. High concentration of suspended materials, both organic and inorganic, does not allow growth of corals because their mouths are clogged by muddy water and hence corals cannot get food and ultimately die of starvation.
Flux of fresh water- flux of huge volume of fresh water into the seas by big continental rivers is injurious for corals growth. This is why corals avoid the vicinity of mouths of major rivers but they thrive around sea islands because there are no big rivers like around Lakshdweep islands of India.

4. Ocean salinity - high salinity of ocean water is injurious for coral growth. The oceanic salinity ranging between 27%o and 30%o is most ideal for the growth and development of coral polyps.
Ocean currents and waves- ocean currents and high energy sea waves are favorable for coral polyps because they bring nutrients to the symbiotic zooxanthellae algae, which are embedded in the outer tissues of coral animals. These embedded algae prepare food through photosynthesis and provide 60 percent of total food requirement of coral polyps.
Submarine platforms - there should be extensive submarine platforms for the formation of colonies by the coral polyps. Such platforms should not be more than 91 meters below sea level.
Pollution of sea water- there should be pollution free coastal water for the survival and growth of corals.

5. Types of coral reefs
The submarine coral reefs are classified in two ways as follows :
On the basis of geology, nature, shape and mode of occurrence as recognized by Charles Darwin during his studies of coral reefs in the pacific ocean.
Fringing reef
Barrier reef
Atoll
On the basis of location
Tropical coral reefs in extensive belts
Marginal belts of coral reefs

6. Fringing reef - Coral reefs developed along the continental margins or along the islands. though fringing reefs are usually attached to the coastal; land but sometimes there is gap between them and land and thus lagoon is formed between the fringing reef and the land. Coral reefs are generally long but narrow in width. The continuity of coral reefs is broken wherever rivers drain into the seas and oceans.

7. Barrier reef - The largest coral reefs parallel to cost line are called barrier reefs. Barrier reefs are the largest, most extensive, highest and widest reefs of all types of coral reefs. The average slope is about 45° but some barrier reefs are characterized by ° slope
E.g.- Great barrier reef.

8. Atoll- A ring of narrow growing corals of horse­shoe shape and crowned with palm trees is called atoll. It is generally found around an island or in elliptical form on a submarine platform. Atolls are found in Antilles sea, red sea, china sea, Australian sea, and Indonesian sea.

9. Origin of coral reefs Stable or stationary land or island
These theories have been based on 3 considerations as follows:
Stable or stationary land or island
Subsiding land or island
Emerging land or island 1 2 3

10. Coral bleaching
The coral bleaching during has been recorded as the most catastrophic event as it accounted for large scale death of corals in the tropical oceans of 60 countries and island nations. The following factors are held responsible for coral bleaching at different levels and scales:
The rise in normal temperature of seawater due to anthropogenic factors i.e. Global warming.
The rise in normal temperature of seawater due to strong EL-Nino southern oscillation events,
Outbreak of diseases and epidemics,
Marine pollution
Elevated ultraviolet radiation level,
Local factors like increase in
siltation, salinity, mining activities etc.,