Coral reef

Coral reefs are found in circum-tropical shallow tropical waters along the shores of islands and continents. The reef substrate is mainly composed of calcium carbonate from living and dead corals. Many other invertebrates, vertebrates, and plants live in close association to the corals, with tight resource coupling and recycling, allowing coral reefs to have extremely high productivity and biodiversity, such that they are referred to as ‘the Tropical Rainforests of the Oceans’.  

Corals live in very nutrient poor waters and have certain zones of tolerance to water temperature, salinity, UV radiation, opacity, and nutrient quantities.  

Geographical Conditions Required

1. Corals generally flourish in clear tropical oceans usually between 30°N and 30°S of the equator.

2. They grow best in the brightly lighted water about 5 to 10 meters deep. The suspended particles interfere with feeding.

3. Corals live in saline water (27%).

4. Coral reef can from to depth of 90 meters, but growth rate declines rapidly after 5 to 10 meters depths. 

5. The reef building corals are found within the 21°C isotherm.

6. Corals are not near the mouths of rivers.

7. Temperature below 18°C causes their death.

8. Individual coral organisms are however, found in some cold, high latitudes waters as well (Norway and Cap Verde Island and off New Zealand and Japan)

Zooxanthellae live symbiotically within the coral polyp tissues and assist the coral in nutrient production through its photosynthetic activities. These activities provide the coral with fixed carbon compounds for energy, enhance calcification, and mediate elemental nutrient flux. The host coral polyp in return provides its zooxanthellae with a protected environment to live within, and a steady supply of carbon dioxide for its photosynthetic processes. The symbiotic relationship allows the slow growing corals to compete with the faster growing multicellular algaes because the tight coupling of resources and the fact that the corals can feed by day through photosynthesis and by night through predation.   

The tissues of corals themselves are actually not the beautiful colors of the coral reef, but are instead clear. The corals receive their coloration from the zooxanthellae living within their tissues.