The gardens of coral that decorate these shallow tropical waters are not, as they appear, plants. They are built up by aggregations of tiny tentacled animals called polyps, which battle through life against a host of enemies, including crustaceans, protozoans, reef fish, starfish and the human species.
Polyps need only three things to survive: food, oxygen and warmth. They feed happily on plankton, receive oxygen (up to 50m below the surface) from the sun, and thrive in clear salty water where the temperature is between 20 degC and 30 degC.
Like other animals, polyps secrete and have sex and herein lies the secret of the reef. After extracting the soluble calcium ions from the ocean, a polyp excretes a cup-shaped limestone skeleton, in much the same way as an oyster secretes its protective shell. This miniscule cup, produced by billions of polyps, accounts for the rocklike texture of the reef.
It is the fascinating process by which polyps reproduce that explains the variety of shapes, sizes and colours along the reef. As the new moon arrives during the winter months, and the full moon during the summer months, the polyps engage in sexual and asexual encounters, producing planula, or offspring, by the billions, Although many of them die as they are born enough survive to perpetuate the species and the reef. The survivors are washed to and fro by the ocean currents, then attach themselves to a solid foundation a submerged volcano or a coastline and secrete and reproduce, eventually forming a colony of polyps. Over a millennium, the colony grows larger and more complex, until a thin ribbon of reef runs parallel to the foundation.
The reef, or colony of polyps if you will, survives best on the seaward side. Elsewhere particularly between the reef and the shore, the colonies die off due to lack of food, or under attack from lifelong enemies, such as parrotfish.