Coral reef
FOCUS QUESTIONS
What are coral reefs?
List and examine the types of coral reefs? How are coral reefs formed?
Where are coral reefs located?
Does the coral reef eat?
CORALS ARE related to sea anemones and they all share the same simple structure, the polyp. The polyp is like a tin can opened at just one end: the open end has a mouth surrounded by a ring of tentacles. The tentacles have stinging cells, called nematocysts, that allow the coral polyp to capture small organisms that swim too close.
Shallow-water corals that live in warm water often have another source of food, the zooxanthellae. These single-celled algae photosynthesize and pass some of the food they make from the sun’s energy to their hosts and, in exchange, the coral animal gives nutrients to the algae. Coral reefs are some of the most diverse ecosystems in the world, housing tens of thousands of marine species. About one-third of all marine fish species live part of their lives on coral reefs. The total area of the world’s coral reefs amounts to less than one-quarter of 1% of the entire marine environment.
Coral reefs can be found around the world, and even in some places that you would not expect. In recent years, scientists have discovered cold-water coral reefs off the coast of Norway and deep underwater in the Mediterranean Sea.
WHERE ARE CORAL REEFS LOCATED?
Coral reefs need warm, shallow water to form.
They form close to the equator, near coastlines and around islands throughout the world.
A significant portion of the world’s coral reefs is located in Southeast Asia and near Australia.
The largest coral reef is the Great Barrier Reef, located off the coast of Queensland, Australia. The Great Barrier Reef stretches for 2,600 miles.
ZONES OF THE CORAL REEF
After a period of time, coral reefs develop zones.
Each zone is inhabited by different kinds of corals, fish and ocean life.
Shore or inner reef zone – This area is between the crest and the shoreline. Depending on the shape of the reef, this area can be full of life, including fishes, sea cucumbers, starfish and anemones.
Crest reef zone – This is the highest point of the reef and where the waves break over the reef.
Fore or outer reef zone – As the reef wall falls off, the waters get calmer. Around 30 feet deep, one will generally find the most populated part of the reef along with lots of different types of coral species.
DOES THE CORAL REEF EAT?
Since polyps need to eat to stay alive, you can think of the coral reef as eating too.
They eat small animals, called plankton, as well as algae. The algae get their food from the sun by using photosynthesis.
This is why coral reefs form close to the surface of the water and in clear water where the sun can feed the algae.
Most reef scientists generally recognise three MAJOR types of coral reefs.
Fringing reefs
Reefs that grow directly from a shore.
Grow near the coastline around islands and continents. They are separated from the shore by narrow, shallow lagoons. Fringing reefs are the most common type of reefs that we see.
A fringing reef runs as a narrow belt [1-2 km wide]. This type of reef grows from the deep sea bottom, with the seaward side sloping steeply into the deep sea. Coral polyps do not extend outwards because of the sudden and large increase in depth.
BARRIER REEFS
This type of reef resembles a fringing reef, but is located further from the shore and can be much bigger than fringing reefs.
Barrier reefs are extensive linear reef complexes that parallel a shore, and are separated from it by lagoon.
A lagoon – as used in the context of coral reef typology – refers to a comparatively wide band of water that lies between the shore and the main area of reef development, and contains at least some deep portions.
Atolls
These circular or horseshoe-shaped reefs encircle a lagoon. No apparent landmass is normally associated with an atoll. The lagoon has a depth of between 80 and 150 metres and may be joined with sea water through a number of channels cutting across the reef.
Atolls are located at great distances from deep-sea platforms, where the submarine features may help in formation of atolls, such as a submerged island or a volcanic cone, which may reach a level suitable for coral growth.
An atoll may have any one of the following three forms.
1. True atoll – a circular reef enclosing a lagoon with no island.
2. An atoll surrounding a lagoon with an island.
3. A coral island or an atoll island which is, in fact, an atoll reef, built by the process of erosion and deposition of waves with island crowns formed on them.
The reef types are differentiated based on large-scale reef morphology, the size and shape of a reef, and its relation to nearby land (if any).
Patch reef – these outcrops of coral usually lie within a lagoon.
Fringing reefs, barrier reefs and atolls are thought to represent stages in coral reef formation. Charles Darwin first proposed this theory of coral reef formation in 1842.
Step 1: A fringing reef forms first, and starts growing in the shallow waters close to a tropical island.
Step 2: Over time, the island subsides and the reef grows outwards, and the distance between the land and the reef increases. The fringing reef develops into a barrier reef.
Step 3: If the island completely subsides, all that is left is the reef. The reef retains the approximate shape of the island it grew around, forming a ring enclosing a lagoon. Darwin speculated that underneath each lagoon should be a bed rock base – the remains of the original island. Subsequent drilling into atolls proved this prediction true.
proved this prediction true.
SUMMARY
Coral reefs are built by and made up of thousands of tiny animals – coral ‘polyps’ – that are related to anemones and jellyfish.
Polyps are shallow-water organisms which have a soft body covered by a calcareous skeleton. The polyps extract calcium salts from seawater to form these hard skeletons. The polyps live in colonies fastened to the rocky seafloor. The tubular skeletons grow upwards and outwards as a cemented calcareous rocky mass, collectively called corals.
When the coral polyps die, they shed their skeleton [coral] on which new polyps grow.
The cycle is repeated for over millions of years, leading to accumulation of layers of corals [shallow rock created by these depositions is called reef].
These layers at different stages give rise to various marine landforms. One such important landform is called coral reef.
Coral reefs, over a period of time, transform or evolve into coral islands, e.g., Lakshadweep.
The corals occur in different forms and colours, depending upon the nature of salts or constituents they are made of.
Small marine plants (algae) also deposit calcium carbonate, contributing to coral growth.
The next lesson will be on the conditions necessary for the successful formation of coral reefs: water – depth, salinity, temperature, turbidity, the presence of beneficial algae and fish.