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Wonders of the Seas: CORAL REEFS


© Jonathan Bird/Oceanic Research Group, Inc.

Over half a billion years ago, before there was any life on land, the seas contained primitive coral reefs, consisting of sponges and primitive corals. This means that coral reefs are among the oldest complex natural communities still in existence on Earth. While many changes and extinctions in reefs have occurred throughout their history, reefs have survived. In fact, 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. People often refer to coral reefs as “rainforests of the sea.” I can’t help but think that a better expression would be to refer to rainforests as “coral reefs of the land.”

A dive on a coral reef is a voyage to another world. The surrealistic landscape is shaded in blue and surrounded by life. The coral reef is a gathering place in the ocean. It is 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-- colonies 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 that produce the huge reef structure.

Corals are members of a group of animals called cnidarians, a phylum that also includes the jellies (jellyfishes) and anemones. All cnidarians have powerful stinging cells called nematocytes to capture prey. While certain jellies can sting people seriously with their potent nematocytes, few species of coral can hurt humans. However, their tentacles represent a real threat to small swimming fish larvae, worms, crustaceans and other small drifting planktonic organisms.

A coral reef is a hard structure made by the combined efforts of thousands of coral polyps. A coral polyp is a tiny animal, with a circle of tentacles around its mouth. Both anemones and coral polyps have similar construction. They are sessile, benthic animals, meaning that they live on the bottom, fixed in one position. Unlike anemones, which are solitary animals, coral polyps grow in colonies. These colonies can become quite large. For example, a brain coral colony can reach over 10 feet across. It may contain many thousands of individual polyps. A coral reef is made up of many coral colonies all living together. The reef may stretch hundreds of miles across, but it is constructed by polyps only a quarter of an inch or less in size.

The reef is built up on a skeleton that the coral polyps make of calcium carbonate (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 coral polyps extract calcium from the seawater and combine it with carbon, produced as a by-product of respiration, to produce the limestone. Hard corals, also called reef-building 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. Staghorn corals can grow as fast as 6 inches a year, under the right conditions, making them among the fastest growing corals in the world. Most other corals grow less than an inch per year, making them slow to recuperate from reef damage. Judging by the size of many of these slow-growing coral colonies, we can 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 the sea whips. It is easy to see how sea whips could be mistaken as plants because of their decidedly bush-like appearance. Brilliantly colored sea fans are another type of soft gorgonian coral.

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

The spicules of soft corals contribute to reef growth along with the hard corals, because when the soft corals die, their spicules end up on the reef. In fact, many other reef animals and plants contribute to reef accretion (positive growth). One of the most significant are coralline algae, encrusting plants which produce thin layers of calcium carbonate. More than just pretty and colorful, this purple to pink-colored alga cements the reef together and contributes to growth.

A few varieties of small corals live in cold waters, but reefs cannot grow there. Biologists believe that cold water inhibits the coral’s ability to form associations with zooxanthellae which facilitates calicification essential for building reefs. 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, tiny drifting plants like diatoms, algae and dinoflagellates, which form the base of the ocean food chain by converting sunlight and nutrients into organic matter. The limited amount of nutrients in the Caribbean limits the growth of phytoplankton, which limits the number of zooplankton (animal plankton) which feed on the phytoplankton. As a result, the waters of the Caribbean are clear, unclouded by plankton. In an ocean with very low food resources, all forms of life have to struggle for survival, and coral is no exception.

Because of the low plankton density, many corals simply cannot get enough nourishment by catching plankton with their nematocyte-laden tentacles. Many species of coral have solved this problem with a symbiotic relationship. These corals, called hermatypic corals, have multitudes of tiny, single-celled plants (actually, a type of dinoflagellate) living in their tissues. The plants, called zooxanthellae (zoo-zan-THEL-eye), give many corals their greenish to brownish color, making the first time observer sometimes think that the coral itself is a plant.

The zooxanthellae, like all plants, have chlorophyll and utilize sunlight to produce sugars for their own energy needs. However, the coral shares in this production, and gets some of its energy from excess sugar production by the zooxanthellae. The coral may also absorb some of the oxygen given off during photosynthesis. In return, the plants use the carbon dioxide and waste products given off by the coral to fuel their production. It’s an ideal mutually symbiotic relationship, and it works well except for one limitation: the sun. Hermatypic coral must be exposed to sunlight for photosynthesis to occur. This limits the depth to which hermatypic corals can live to less than about 75 meters (250 feet).

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.

For a new coral reef to form, a coral larva has to settle down on an appropriate surface and start to grow. A solid foundation for a new coral reef could be as simple as a small rock on the sand, or a barren piece of dead coral skeleton. 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.

In many places all over the Caribbean, artificial reefs are created by the intentional sinking of old ships onto barren patches of underwater terrain. In only a few short years, corals cover the wrecks, and fish seek the new reef for shelter. Within just a few years, soft corals and sponges will grow on the wreck. A hundred years down the road, 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 itself 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. All this and more is done by tiny animals less than a centimeter across. Perhaps this is why I believe that the coral reef truly is a living wonder.

Coral as seen from the surface in shallow water, Malaysia.

Brain coral polyps
Close-up of interconnected polyps of Brain coral. The green zooxanthellae are quite obvious in this shot!

Star coral polyps
Close-up of star coral polyps

Soft coral
Soft coral on a wall, Philippines.

Soft coral
Close-up of soft coral polyps and spicules.

Indonesian Reef
A colorful Indonesian reef.

Star coral spawning, Caribbean.

All images on these pages for non-profit educational use only.

update 2/10/13