Coral Triangle

Crinoids, a type of sea star, often grip soft corals and feed alongside them in fast, strong currents. Fiji
© Cat HOLLOWAY / WWF-Canon

Tropical coral reefs are found between 30° north and south of the equator, in areas where surface water temperatures do not drop below 16°C. The total area of the world's tropical coral reefs is around 284,300km².

These reefs can grow up to 100 centimeters per year and can form huge structures over incredibly long periods of time, making them the largest and oldest living systems on earth. For example, Australia's 2,000-km long Great Barrier Reef was formed over the course of five million years.

Tropical coral polyps have small algae, or zooxanthellae, growing inside them. This is a cooperative, or symbiotic, venture. The algae get shelter and food (in the form of nutrients from captured plankton) from the polyp, while the polyp also gets some food in return from the algae via photosynthesis (turning light energy from the sun into food). This photosynthesis means algae need sunlight to live, and this is why tropical corals only grow where the sea is shallow and clear.

Delicate branching coral colonies are the most susceptible to coral bleaching, storm pounding and anchor damage. Fiji
© Cat HOLLOWAY / WWF-Canon

The algae also give corals their colour. If the algae become stressed, such as if the water temperature becomes too high, they leave the polyp. This exposes the white, calcium carbonate skeletons of the coral and is what we call coral bleaching.

Tropical coral reefs form the basis of complex ecosystems. Their soft polyps provide a ready food source for many other sea creatures, while reef caves and crevices are first-rate locations for breeding or hiding from predators. Up to 4,000 individual species can co-exist on a single reef at a density 100x greater than the average for the open ocean. These include the coral species themselves as well as algae, fish, dugongs, marine turtles, sea snakes, worms, crustaceans, mollusks, starfish, and many, many more.