Coral Reefs Types and Theories of Origin

Dr. Vijendra Kumar Pandey

UNIT - V

Corals

Landlords of the Reefs

Hard Corals

The Reef Builders

• Polyps build hard limestone cups around their bases

• The cups cement together to make a coral colony

• Reefs are made of thousands of hard coral colonies next to and on top of each other

• Tentacles release stinging cells when something brushes by them

• Polyps make their own limestone cup to hide in during the day

• At night, polyps come out to catch plankton floating by.

What’s a Polyp?

Symbiosis:

So Happy Together

• Two organisms living together and helping each other is called symbiosis.

▫ Zooxanthellae make oxygen, remove the polyp‟s wastes, and make food for the polyp from photosynthesis.

▫ Coral polyps protect the zooxanthellae, release CO2, and provide it with necessary nutrients from their own waste.

Features of Coral reefs:

• Coral reef and atoll are significant submarine features - These are formed due to

accumulation and compaction of skeleton of lime secreting organisms known as

„Coral polyps‟.

• Coral reefs are formed by shell over shell accumulation.

• „Coral polyp‟ cannot live above the water level.

• They are generally attached to submarine ecosystem.

• Coral reefs have diversity of 10,00,000 species. These are also called rainforest

of oceans.

Location of Reefs

Found between 30°north and 30°south

Reason: coral reefs do not thrive in areas where the surface

temperature falls below 70°F

Two areas of coral reefs

– Continental

– Oceanic

• Indian

• Pacific

Found mainly in “Tropical Oceans” (Temperature range 21 – 27 ℃)

Coral don‟t live deeper in water, and above sea level also. - They don‟t get sunlight and oxygen below 91 m, so.

Clean and sediment- free water is required to grow polyps. - Mud will clog the mouth of polyps otherwise.

Fresh water is also not good for their growth due to lack of minerals in it.

High salinity of water is bad for their growth. Requires 27°/ ₀₀ to 30° / ₀₀ Salinity.

Ocean currents and waves bring food for polyps. Inside Lagoons there will be dead polyps.

Polyps need marine platforms to grow below the sea level.

Conditions for growth of Coral reefs

Coral reefs are classified in two ways:

1) On the basis of nature, shape, & mode of occurrence

i. Fringing reefs

ii. Barrier reefs

iii. Atoll

2) On the basis of location

i. Tropical Coral reefs

ii. Marginal belt coral reefs

Type of Coral Reefs

1. Fringing reefs

• Reefs developed along the continental margins or along the islands are called fringing reefs.

• Usually attached to the coastal land. • But some times there will be gap

between land and reef, forms shallow water zone, called „boat channel‟.

• Reefs are narrow in width but long. Breaks at river mouth.

2. Barrier reefs

Barrier Reefs - Separated from land by lagoons • The largest coral reefs off the coastal

platforms but parallel to them are called „barrier reefs.‟

• There is extensive but shallow lagoon between coastal land and barrier reef.

• These reefs are continuous. • Some times there will be tidal inlets -

These reefs require 91m depth for development - Ex: Great Barrier reef, parallel to the east coast of Australia.

3. Atoll

• A ring of narrow growing corals of horseshoe shape around the island shape coastal area are called Atolls.

• The depth of lagoons ranges from 240 to 420 feet. • Three types of Atolls are found:

i. True atoll – Circular reef enclosing shallow lagoon, without island.

ii. Island atoll - Circular reef enclosing shallow lagoon, with island. iii. Coral island atoll - Island is not present initially, but formed later

on, due to deposition. True atoll is Island atoll.

References/Further Reading

1. Tom S. Garrison - Essentials of Oceanography.

2. R.C Sharma & M Vatal - Oceanography for Geographers.

3. Savindra Singh – Physical Geography