The Coral Reef: Where Do They Come From?

The clear, warm waters of the tropics are home to perhaps the most dramatic environment on earth — the coral reef. Here we find the most breathtaking groups of animals, both in terms of their incredible colors and sheer numbers.

The reefs themselves are constructed by countless coral polyps — relatives of anemones and jellyfish. Each polyp lives as a free-swimming larva before abandoning the water column and settling on the reef. When it does so, the polyp forms its own protective, cuplike, calcium skeleton around itself, where it can retreat if threatened. Once settled, the polyp begins to reproduce asexually, growing and expanding to form a colony of thousands, or even millions, of connected clones until it reaches sexual maturity.

How do single polyps build into a reef?

In hard, reef-building corals, each generation of polyps builds upon the last, so that the reef itself has a coating of living polyps on a core of the dead limestone skeletons of their forebears. This layer-upon-layer building can, over time, be sufficient to form colossal geological structures, such as Australia's Great Barrier Reef. This tendency of tiny polyps to construct an entire habitat, providing food and shelter to a whole host of other animals, is the reason that reefs are so amazingly populous and diverse. Soft corals also settle as polyps on surfaces, but do not construct the same kind of limestone skeletons. Instead, their skeleton is reduced to supporting rods throughout their tissue. Despite this, soft corals can still produce incredible and dazzling structures, often branched and coated in brilliantly colored polyps.

What conditions do reef-building corals require to grow?

The warm waters of the world's tropics are not the only places where corals can grow, but they are the home of the reef-building corals. In order to flourish, these corals require warmth and bright light. This usually means ocean temperatures of 68-82°F (20-28°C) and shallow waters, where the sunlight can penetrate to feed the symbiotic algae that live in the coral's tissue. For this same reason, the coral needs clear water — suspended matter scatters light — so healthy reefs are seldom found near estuaries on nowadays, too near industrial cities. Reefs also benefit from wave action and water currents which not only deliver food and nutrients to the polyps, but also prevent excessive sedimentation on the reef.

Perhaps because of the fact that they live in comparatively nutrient-poor waters, corals have evolved to be highly efficient. Not only do they harness the sun's energy, they can take calcium to construct their skeletons directly from the water and their symbiotic algae can recycle animal wastes in order to photosynthesize. In doing so, the algae produce the sugars needed both by themselves and the coral.

Where in the world are coral reefs found?

Coral reefs are found in the belt encircling the globe known as the tropics. The seas found here are characterized by stable temperature and light conditions, as well as bright sunshine. Reefs occur in the Caribbean, the Red Sea. and across the Indo-Pacific. They can also be found beyond the tropics, for example in southern Japan, where currents carry warm waters to these shores, creating the necessary conditions for reef growth.