Nearly half a billion years ago, before there was any life on land, the seas contained primitive coral reefs, very closely related to the ones still alive today. Coral reefs are the oldest natural communities still in existence on Earth. Some coral reef animals known today are almost unchanged from those found in fossils dating from the age of dinosaurs, 100 million years ago.

Coral reefs are spectacular to behold, lush gardens in the sea, supporting a staggering amount of marine life in a densely packed, thriving marine metropolis. In fact, coral reefs have the largest abundance and greatest diversity of life living together of any place on Earth, including the tropical rain forests. Every major phylum of animals from bacteria and sponges to fish, reptiles, birds and mammals are all present on and around coral reefs.

Join us as we dive into the depths of the Caribbean and South Pacific Oceans to discover the secrets of the beautiful and life-giving coral reefs thriving in this tropical garden of Eden.

A dive on a coral reef is a voyage to another world. The surrealistic landscape is shaded in blue and surrounded by life...Life in a thousand forms. The coral reef is a gathering place in the ocean. An oasis in a desert, a place which gives shelter and food in an ocean where these things are rare. In fact, the entire tropical ocean ecosystem depends on the reef for sustenance. The reef itself is a living, growing organism- a colony of tiny animals all working together to create the largest structures on Earth. This is one of the most complex and mysterious ecosystems known to mankind, and it all works because of the tiny animals which produce the huge reef structure.

Our introduction to the coral reef begins with a very unusual, but related, animal, called an anemone. An anemone looks like a flowering plant, but it is in fact an animal, an invertebrate which lives on the ocean bottom waiting for some unsuspecting creature to come along and fall prey to its arrangement of stinging tentacles. These tentacles get their sting from a structure they contain called nematocysts. There are millions of these stinging cells in each tentacle of the anemone, making it a very effective, if stationary, predator.

Anemones are members of a group of animals called Cnidarians, a phylum which also includes the jellyfish and corals. All Cnidarians have nematocysts to capture prey, and perhaps no Cnidarian is more well known than the jellyfish, with its powerful stinging tentacles.

A coral reef is a hard structure made by the combined efforts of thousands of coral polyps. A coral polyp is really just a tiny anemone, with tentacles and nematocysts to capture prey. But unlike anemones, coral polyps grow in colonies. These colonies can be quite large. For example, this brain coral colony is over six feet across! It contains many thousands of individual polyps. A coral reef is made up of many coral colonies all living together. The reef may be hundreds of miles across, but it is made by polyps the size of a pearl.

The reef is built up on a skeleton that the coral polyps make of limestone. Ever so slowly, over hundreds, or thousands of years, the coral polyps add limestone to their skeletons in layers, and grow outward and upward, expanding the reef.

The limestone is made of calcium carbonate. Calcium is extracted from the seawater and combined with carbon, produced as a by-product of respiration, to produce this limestone. Hard corals produce reefs with their rigid, rock-hard skeletons. Although living coral hides its skeleton under living tissue, a dead piece of coral reveals just the white skeleton.

The process of growing can be fairly fast, or painstakingly slow, depending on the coral. This damselfish has made its nest (which it fiercely defends) within a colony of staghorn corals. Staghorn corals can grow as fast as 15 centimeters a year, under the right conditions, making them among the fastest growing corals in the world. Most other corals grow only one or two centimeters per year, making them slow to recuperate from reef damage. Judging by the size of many of these slow-growing coral colonies, it is reasonable to predict that some of them are well over one thousand years old!

Not all corals are so-called hard corals, producing a rigid, boulder-like reef. There are many species of soft corals, which look like trees, or bushes, flexing in the currents. Many a snorkeler or diver has mistaken these animal colonies for plants, but they are colonial animals just like the reef building corals. There are many varieties of soft corals, including not only the branching soft corals but also the gorgonians like this sea whip. It is easy to see how sea whips could be mistaken as plants because of their decidedly bush-like appearance. This brilliantly colored sea fan is another type of soft gorgonian coral.

For a skeleton, generally soft corals make tiny needle-like splinters out of calcium carbonate, called spicules. These spicules are embedded in the colony to give it strength, yet still allow the colony to flex and bend with the currents.

A few varieties of small corals are found in cold waters, like this North Atlantic soft coral, but reefs cannot grow in cold water. Scientists believe that cold water inhibits the coral's ability to excrete calcium carbonate. Coral reefs are therefore only found in ocean regions where the water is always above approximately 65 degrees Fahrenheit, or roughly between the tropic of cancer to the north and the tropic of Capricorn to the south.

In an area with this much diversity and life, it is easy to think that the tropical oceans are highly rich in nutrients. This is the popular misconception. However, compared to the cold, murky waters of the temperate seas, coral reefs live in nearly sterile water. The green murkiness of cold water is caused by a large amount of photosynthetic phytoplankton which provides a base for an extensive and highly productive food chain.

The clear water of the tropics is a result of the minimal amount of plankton suspended in the water. Plankton, the base of the food chain in all oceans, cold or warm, is very low in density in the tropics. In an ocean with very low food resources, all forms of life have to be clever to survive, and coral is no exception.

Because of the low plankton density, many corals simply cannot get enough nourishment by catching plankton with their nematocyst-laden tentacles. In order to get additional nutrients, many corals have become symbiotic with a form of algae, called zooxanthellae.

Zooxanthellae are single celled green plants which live in the skin tissue of the coral polyps, giving them a plant-like green tint. During the day, the abundant tropical sunshine allows the zooxanthellae to photosynthesize and produce sugars, which are shared with the coral polyps. In exchange, the coral’s respiratory by-product of carbon dioxide is shared with the zooxanthellae. Together, they live in harmony, each unable to survive without the other.

For the reef to grow in size, more polyps are created by asexual division, a process known as fission. A polyp undergoing fission simply breaks itself in two, and each half grows back into a normal polyp. But this is not the only way coral can reproduce. In order for new colonies to form far away from the existing reefs, the corals produce planktonic larvae to drift with the current and settle elsewhere. This is done with sexual reproduction.

There are two types of sexual reproduction in coral. Some corals are called brooders. These corals produce a planktonic larva called a planula inside of each polyp. The egg is fertilized internally, and incubated until the planula is ready to be released.

Other corals fertilize externally, with some polyps releasing eggs and others releasing sperm. The eggs are then fertilized in the water column, and develop into larvae. Obviously, this requires some form of timing so that the eggs and sperm are released at the same time. The coral polyps synchronize themselves to the phases of the moon, so that they all release their eggs and sperm on the same night. The corals spawn at night to reduce the number of larvae which are eaten by predators. Even with this precaution, very few of the planktonic larvae will live to form a new reef.

The process of coral spawning has only recently been discovered, and is still a bit of a mystery. On a reef in the Florida Keys, a scientist takes samples of spawn for research purposes. Here, another researcher uses a special trap to catch spawn for a fertilization experiment. It is important for scientists to learn more about coral because of the crucial role it plays in the ecosystem.

The importance of the coral reef in the tropical ocean ecosystem cannot be stressed enough. Coral reefs provide nesting areas and hiding spots for fish and invertebrates, making the reef into a teeming metropolis of life. This, in turn, attracts larger predators to the area looking for food. A coral reef may be packed with fish and other marine life, while only a hundred yards from the reef, there is hardly a thing to be found.

Here, a piece of marine debris out on a sandy bottom has served as a foothold for a small colony of coral. The coral colony has in turn become a hiding spot for a school of Blue Chromis. As the cameraman approaches the school, they take refuge in their protective mini-reef. As the reef grows larger, more fish and invertebrates will be attracted to it. In time, a giant new coral reef community will arise from these humble beginnings.

A solid foundation for a coral reef could be as simple as a small rock on the sand. Sometimes, however, the most unlikely objects can form the basis for the growth of a reef. Many man-made objects from oil rigs to shipwrecks start new reefs by providing a foothold. After a few years in the sea, even the largest ships begin to fade away as they are covered by corals and obscured under a biological blanket of marine life. In time, the metals will corrode away completely, leaving behind just a reef. This Japanese cargo ship sank during world war two in the Pacific, its forward deck gun now serving as the foothold for several coral colonies, and a small school of fish.

In many places, like the Florida Keys, artificial reefs are created by the intentional sinking of old ships onto barren patches of underwater terrain. In only a few short years, corals will cover the wrecks, and fish will seek the new reef for shelter. Within a hundred years, the coral will completely overgrow the ship, creating a beautiful and life-sustaining coral reef to endure for thousands of years, long after the ship has rusted away.

Coral reefs are among nature's greatest spectacles. They form communities of startling complexity and help to create an entire ecosystem in oceans with low nutrient resources. Throughout their life stages, corals act as food for other animals, shelter for other animals and producers of the greatest examples of natural architecture in the world. They can even bring new life to man’s disaster. All this and more is done by tiny animals only a centimeter across. Perhaps this is why we say that the coral reef truly is a living wonder.