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At Water's Edge (AWE)

Coral Reefs


Many people believe that coral reefs are plants, but that is very false! Coral reefs are actually huge ecosystems, made up mainly of tiny animals called coral polyps. Polyps are simple creatures, related to jellyfish and anemones, and they are basically just stomachs with tentacle-bearing mouths at the top and calcium carbonate exoskeletons.




     Cross-section of a Coral Polyp, taken from the Australian Marine

Environment Protection Assocation website

Fun Coral Facts!

  • Coral polyps are translucent, meaning they are essentially colourless. So how do corals get the vibrant hues they are best known for?

  • Corals host algae called zooxanthellae, which provide the wide array of colours we see in reefs. The two have a symbiotic relationship that they both benefit from. The algae receive protection from the polyps and are also provided with materials for photosynthesis. The coral then uses the photosynthetic byproducts as food, which provide it with all the nutrients it needs for growth and reproduction.

  • Corals are carnivorous! In addition to the food received from the zooxanthellae, corals sometimes catch zooplankton and even small fish. At night, they extend their barbed, venomous tentacles and capture their prey. Nematocysts are what give the tentacles their stinging power. 

  • A polyp's lifespan can be anything from two years up to hundred of years!

  • A group of coral polyps is called a colony.

  • Coral reefs form over centuries as colonies grow and die, leaving behind their limestone exoskeletons, which the next generation of polyps attach to and grow. Over time, the layers of calcium carbonate build up, forming the base of the reef, and the present colony live atop that, providing food and homes for a variety of marine creatures.


Importance of Coral Reefs

  • Coral reefs are an important ocean ecosystem, second only to rainforests in terms of biodiversity. Many of the ocean's cretaures depend on the reefs for food and homes, especially when young.

  • Their role as nursery makes coral reefs vital to the world's fisheries. About a billion people worldwide depend on fisheries for food and/or revenue, making coral reefs essential to maintaining persons' livelihoods, especially in small islands like Grenada.

  • Coral reefs, as well as mangroves, provide the important service of coastal protection. They break the power of the waves during storms, hurricanes, and even tsumanis, and help to prevent coastal erosion, flooding, and loss of property on the shore.

  • Tourism is also largely dependent on coral reefs, especially in the Caribbean. Their vibrant colours and large assortment of fish and other marine creatures make reefs a prime tourist attraction. They are also responsible for white, sandy beaches and calm, pristine waters. Not only does this contribute to national GDP, but sustainably managed coral reef-based tourism can also provide significant alternative or additional sources of income to poorer coastal communities .

  • Several advances in medicine have been made using extracts of coral. Treatments for asthma, arthritis, heart disease and even cancer have been developed from coral reef organisms.

  • Coral reefs also have an intrinsic value, not just for their mesmerizing beauty and the joy one can derive from swimming among them, but also to the coastal communities that depend on them, in more ways than one. 





















Conditions for Healthy Coral Growth

  • Sunlight: the zooxanthellae in corals need sunlight in order to photosynthesize, so by extension, corals need sunlight to survive. Because of this, corals rarely grow below 65m from the surface, as light penetration decreases with depth. The optimum depth is 20-40m.

  • Warm water: Depth is also important in terms of water temperature; it gets colder with depth, and so corals cannot live below a certain depth. Different species of coral can withstand different temperature levels, but generally they live in warm water between 20 and 32° C. The optimum water temperature is  23-29° C.

  • Clean, clear water: 

    • Turbid or cloudy water can block sunlight penetration and be a hindrance to photosynthesis

    • Sediments (from rivers for example) can cloud the water and settle on the corals, choking them

    • Wastewater and other pollutants may contain nutrients which cause eutrophication and other harmful phenomena

  • Saltwater: corals require salinity - and a particlar balance of it - to thrive. This, along with the sediment reason given above, is why corals do not grow in or near estuaries.



Corals are very sensitive, as is obvious from the specific conditions needed for survival. They have narrow tolerance ranges, and so the smallest changes in temperature or depth or salinity can be detrimental to the life of the reef. As corals become stressed, the zooxanthellae flee, and the coral loses its colour; at this point, it is "bleached", and death follows soon after. 

Coral reefs are the most important marine ecosystem, but they face many, many threats, both natural and anthropogenic.


  • Natural

    • Coral diseases

    • ​Global Warming

    • Sea Level Rise

    • Ocean Acidification

    • Storms

    • Ozone Depletion

    • Predation

  • ​​​Human

    • Introduction of invasive species

    • Coastal development

    • Unsustainable agricultural practices, and by extension, Sedimentation

    • Pollution - chemical, thermal and otherwise

    • Destructive fishing methods and overfishing

    • Overharvesting (coral mining)

    • Careless tourist action

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