Coral reefs are one of the most productive natural ecosystems in the world. Coral are small
benthic animals about the size of an ant. Reef building corals are colonial animals which
secrete a hard skeleton of CaCO3 around their bodies. Their distribution throughout the
ocean floor is general but reef corals are restricted to shallow water areas with average
monthly temperatures >18ºC (64ºF), normally in the tropics (but because of warm ocean
currents can approach 35ºN & 35ºS on the western margins of ocean basins). Water
warmer than 30ºC (86ºF) will cause them to die, such as during El Niño (when the trade
winds weaken or reverse & result in a southward flowing nutrient poor current of warm
water off the coast of western South America). Besides coral, algae, molluscs &
foraminifera contribute to the reef structure. They are restricted to shallow water (< 150m)
(mostly 0-50m) surrounding continents & islands because they have a symbiotic
(association of dissimilar organisms to their mutual advantage) relationship with a small
green algae. It lives in the coral tissue & makes photosynthetic products directly available
to the coral & in return gets nutrients & CO2. Reef growth also requires a relatively normal
salinity & free from particulate matter. They are widely developed in the Pacific & Indian
Oceans & Caribbean Sea.
Types of Reefs
1) Fringing - at the margin of a land mass, contains a small amount living coral with the
greatest concentration seaward since it is more protected from salinity & sediment
changes; ex: Tahiti, Hawai’i, O’ahu, Rarotonga
2) Barrier - linear or circular developments separated by a lagoon from the land mass. It is
believed that as the substratum of a fringing reef subsides, a barrier reef develops.
Best example is the Great Barrier reef - 150km wide & 2000km long; 2nd largest is
off the coast of Belize; also Bora Bora, Truk, Clipperton
3) Atoll - form either on a subsiding continental shelf or around the margin of a sinking
volcanic island. Most occur in the equatorial Pacific; ex: Bikini, Eniwetok,
4) Patch Reef/Reef Pinnacle - grow separately from other reefs, usually in a lagoon;
shaped like truncated cones or pinnacles
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