Coral reefs may look as if they were made of stone, but in fact they are the creation of very sensitive creatures. The animals that make up a coral reef are called polyps – they can be microscopic or large, but no matter what size they are, they are the essential, most basic building blocks of any coral reef.
Polyps are animals that look like a transparent sac, at first sight. The sac that makes up their body has an opening that serves as a mouth and is usually surrounded by tentacles called cnidae or nematocysts, which protect the mouth by secreting stinging substances.
Based on their complexity, polyps can be categorized into two classes: Hydrozoa and Anthozoa. The body of Hydrozoa polyps is fairly simple, consisting mainly of the sac and a small number of tentacles, while the Anthozoa – the type that makes up coral colonies – is much more complex and distinguished by the existence of a stomodaeum, a short tube that links the polyp’s mouth to the sac-like body.
Coral polyps are soft – therefore, they build skeletons made from calcium carbonate around their bodies to protect themselves. A coral reef starts when a tiny, transparent coral polyp attaches itself to the seabed and starts secreting its protective calcium carbonate skeleton, and then it multiplies through division or budding. Coral reefs grow as the polyps proliferate – more polyps means more calcium carbonate skeletons and more extended reef structures. The process of reef formation is very slow, taking hundreds, if not thousands or even millions of years for a large reef to build – there are reefs on Earth today that started growing 50 million years ago and they are still alive and growing.
Coral polyps are transparent and calcium carbonate is white, yet coral reefs are known to be brightly colored. The color we see does not come from the coral itself, but from a type of algae called zooxanthellae that live inside the coral polyp. The relationship between the polyp and the algae is symbiotic. This means an association between the polyp and the algae which is so close that none of them would be able to exist without the other one. During the day, the algae engage in photosynthesis. They harness the energy they get from the sun rays that penetrate the water, and they transform the carbon dioxide into oxygen and glucose necessary for the polyps to survive. Besides producing food for the coral polyps, zooxanthellae also produce pigments that become visible through the tiny, clear polyp bodies, conferring the coral formations their brightly colored appearance. If the process of photosynthesis is halted for some reason or the polyps that act as hosts to the algae are stressed, the algae become exposed or evicted by the polyp and they die, which leads to the disappearance of the reef’s color, too – a process called bleaching. The calcium carbonate skeletons produced by coral polyps are white and, if there are no algae to make them colorful, they become white again.