Coral reefs are made of tiny animals called “polyps” that belong to a group of animals known as Cnidaria. Polyps have a hard carbonate exoskeletons (outer skeleton) which support and protect the coral polyps. They are connected by living tissue to form a community. Stony corals (scleractinians) are the corals primarily responsible for laying the foundations of, and building up, reef structures
The corals depend upon a symbiotic (endosymbiotic*) relationship with algae-like unicellular flagellate protozoa (zooxanthellae) that are photosynthetic and live within their tissues.
*An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis. Zooxanthellae give coral its coloration.
The zooxanthellae require sunlight for photosynthesis. They provide nutrients to the polyps and reduce the level of carbon dioxide. This makes the conditions for the formation of skeletons more suitable. In turn, the zooxanthellae get a suitable habitat.
The coral species which harbour the zooxanthellae are called hermatypic. Cold water corals lack the symbiotic Dinoflagellates. The species without zoooxanthellae is called ahermatypic.
Not all corals are reef building species. There are also hard corals existing as single, solitary polyps. Some temperate species form small colonies only. Corals that lack the hard outer coverings of calcium carbonate are soft corals.
CONDITIONS FOR GROWTH
Corals need to grow in shallow water where sunlight can reach them. Corals depend on the and these algae need sunlight to survive. Corals rarely develop in water deeper than 165 feet (50 meters).
It should be available at suitable depth (45-55m) so that corals can colonise and grow upwardly and outwardly, to form a massive reef
Corals need clear water that lets sunlight through; they don’t thrive well when the water is opaque. Sediment and plankton can cloud water, which decreases the amount of sunlight that reaches the zooxanthellae. Light-absorbing adaptations enable some species to live in dim blue light
Reef-building corals require warm water conditions to survive. Different corals living in different regions can withstand various temperature fluctuations. However, corals generally live in water temperatures of 25–32° C.
Corals need saltwater to survive and they thrive in waters with salinity levels of 27-35 parts per 1000. This is why corals don’t live in areas where rivers drain fresh water into the ocean. Precipitation of calcium necessary for the formation of skeletons. Water temperatures and salinity have to be high and carbon dioxide concentrations have to be low.
Corals are sensitive to pollution and sediments. Sediment can create cloudy water and be deposited on corals, blocking out the sun and harming the polyps. Wastewater discharged into the ocean near the reef can contain too many nutrients that cause seaweeds to overgrow the reef (Eutrophication)
Strong wave action
It ensures: supply of food and oxygen, the distribution of larvae and prevents sediment to settle on the reefs
TYPES OF CORAL REEFS
They 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 reef.
Barrier reefs also parallel the coastline but are separated by deeper, wider lagoons. At their shallowest point they can reach the water’s surface forming a “barrier” to navigation.
Difference between Fringing and barrier reefs
1. Barrier reefs have at least some deep portions; fringing reefs do not.
2. Barrier reefs tend to be much farther away from shore than fringing reefs
Atolls are rings of coral that create protected lagoons and are usually located in the middle of the sea. Atolls usually form when islands surrounded by fringing reefs sink into the sea or the sea level rises around them (these islands are often the tops of underwater volcanoes). The fringing reefs continue to grow and eventually form circles with lagoons inside.
Patch reefs are small, isolated reefs that grow up from the open bottom of the island platform or continental shelf. They usually occur between fringing reefs and barrier reefs. They vary in size, and rarely reach the surface of the water.
1. They are called "rainforests of the sea". Shallow coral reefs form some of the most diverse ecosystems on Earth. They are gene pool centres due to high genetic diversity.
2. They are sensitive bio-indicators. The study of coral reefs is important for providing a clear, scientifically-testable record of climatic events over the past million years or so.
3.Corals are very important in controlling how much carbon dioxide is in the ocean water. Coral polyp turns carbon dioxide in the water into a limestone shell. Without coral, the amount of carbon dioxide in the water would rise dramatically.
4. They protect coasts from strong currents and waves by slowing down the water before it gets to the shore. They provide a barrier between the ocean and the shore.
5. By providing complex and varied marine habitats, they support a wide range of other organisms.
Example : Fringing reefs just below low tide level have a mutually beneficial relationship with mangrove forests at high tide level and sea grass meadows in between: the reefs protect the mangroves and seagrass from strong currents and waves that would damage them or erode the sediments in which they are rooted, while the mangroves and sea grass protect the coral from large influxes of silt, fresh water and pollutants
6. Reefs are home to a large variety of animals, including fish, seabirds, sponges, cnidarians, worms, crustaceans, mollusks, echinoderms, sea turtles and sea snakes etc
7.They can play a role as nursery ground. The fishing industry depends on coral reefs because many fish spawn there and juvenile fish spend time there before making their way to the open sea
8. Revenue from tourism
CORAL REEFS IN INDIA
Gulf of Kutch
Exclusively consists of fringing reefs. The reefs are relatively less developed due to large range of temperature and high salinity. The entire Gulf of Kutch is also a marine national park.
Exclusively coral atolls with 36 islands of which 10 are inhabited.
Gulf of Mannar
Fringing reefs with a chain of 21 islands from Rameswaram in the north to Tuticorin in the south. This part of the gulf forms part of the Gulf of Mannar biosphere reserve.
Andaman and Nicobar Islands
Exclusively fringing reefs of about 500 islands
WORLD DISTRIBUTION – IMPORTANT REGIONS
1. Great Barrier Reef, Coral Sea near Australia
2. Red Sea Coral Reef, Red Sea near Israel, Egypt and Djibouti
3. New Caledonia Barrier Reef, Pacific Ocean
4. Mesoamerican Barrier Reef, Atlantic Ocean near Mexico, Belize, Guatemala and Honduras
5. Florida Reef, Atlantic Ocean and Gulf of Mexico
6. Great Chagos Bank, The Maldives
Coral bleaching is the loss of intracellular endosymbionts ( zooxanthellae) through either expulsion or loss of algal pigmentation. It occurs when the conditions necessary to sustain the coral's zooxanthellae cannot be maintained.
Coral bleaching is a generalized stress response of corals and can be caused by a number of biotic and abiotic factors, including:
1. increased or reduced water temperatures
2. oxygen starvation caused by an increase in zooplankton levels as a result of overfishing
3. increased solar irradiance (photosynthetically active radiation and ultraviolet light)
4. changes in water chemistry (in particular acidification -Acidification affects the corals' ability to create calcareous skeletons, essential to their survival)
5. increased sedimentation due to silt runoff
6. bacterial infections - Black band disease, Skeletal eroding band , White band disease , White pox disease
7. changes in salinity
9. low tide and exposure
10. Cyanide fishing - It is a method of collecting live fish mainly for use in aquariums, which involves spraying a sodium cyanide mixture into the desired fish's habitat in order to stun the fish. The practice hurts not only the target population, but also many other marine organisms. Cyanide concentration slows photosynthesis in zooxanthellae, which results in coral reefs losing color; it also eliminates one of their major food sources.
11.elevated sea levels due to global warming
12. mineral dust from African dust storms caused by drought
While most of these triggers may result in localized bleaching events (tens to hundreds of kilometers), mass coral bleaching events occur at a regional or global scale and are triggered by periods of elevated thermal stress resulting from increased sea surface temperatures.
The coral reefs that are more subject to continued bleaching threats are the ones located in warm and shallow water with low water flow. Physical factors that can prevent or reduce the severity of bleaching are available for the reefs located under conditions that include low light, cloud cover, high water flow and higher nutrient availability
- Storms and tidal emersions
- El Niño: increased sea surface temperatures, decreased sea level and increased salinity from altered rainfall
- Predation by fishes, marine worms, barnacles, crabs, snails and sea stars
- Dust outbreaks
- Chemical pollution
- Land development
- Ocean acidification