Coral Reefs
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Coral Reefs


Most of this planet we call Earth is covered by water -- a vast network of oceans and seas. We have given each body of water a separate name. But in reality, they are all connected -- an immense, watery habitat for a spectacular variety of plants and animals we rarely see, and therefore know little about.

The larger marine creatures -- like whales, dolphins, and sharks -- get most of the attention. But some of the tiniest animals in the sea may be the most remarkable -- and most essential for supporting marine life.

Within the world's oceans, the greatest variety of life is found on amazing living structures called coral reefs. These fragile reefs play a critical role in sustaining a thriving ocean habitat, especially in tropical oceans. They also provide many benefits to humans as well. Yet, coral reefs are built by tiny animals, each smaller than a pencil eraser.

Coral reefs are spectacular to behold, lush gardens in the sea, supporting a staggering diversity of marine life in a densely packed, thriving marine metropolis. In fact, coral reefs harbour the greatest diversity of life in the oceans, and are second only to tropical rainforests in the number of species found in one area on Earth. Nearly 25% of all marine life depends upon coral reefs for their survival, yet reefs make up only a very small part of the world's oceans: far less than 1% of the vast ocean floor. Because of this, coral reefs are often called "the rainforests of the sea".

Nearly 400 million years ago, before there were any animals on land, the primitive ancestors of coral reefs formed in the seas. Today's coral reefs were built up during the last 10,000 years, as the last Ice Age ended and the glaciers receded. Coral reefs are the oldest complex natural communities or ecosystems existing on Earth. Some coral reef animals alive today are virtually unchanged from those found during the age of dinosaurs, 100 million years ago.

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. It's a place which provides a variety of food and shelter in the tropical ocean, where such variety is hard to find. The entire tropical ocean depends upon the coral reef for sustenance.

The reef itself may look like a collection of rocks or boulders, but actually it's a living, growing organism -- a colony of tiny (from less than a millimetre to a few centimetres in diameter) animals called coral polyps. These little polyps all work together to create huge and varied reefs, some of which are the largest structures on Earth, stretching hundreds of miles across. Buccoo Reef is the largest and most famous in Tobago, but there are also reefs at Speyside, Arnos Vale, Englishman's Bay, Charlotteville and many other bays all around Tobago.

The seas around Tobago support rich coral growth because of adequate temperature conditions (seldom below 23-25C), relatively clear water, with little suspended matter. The waters around Trinidad, however, are less salty (less than 30 parts per thousand) owing to admixture with fresh-water from the Orinoco River in Venezuela, and are also heavily silt-laden due to discharges of Trinidad's larger rivers, as well as the Orinoco, particularly during the rainy season. These two factors are mainly responsible for the paucity of coral reef formations around Trinidad.

Much like their relative the sea anemone, coral polyps have sticky tentacles with stingers (see The Amazing Cnidoblast ) to catch passing prey for food. At night, the polyps feast on small floating organic material called plankton, which populate the oceans. But their primary source of food is microscopic plant cells called zooxanthellae that actually live within the tissue of hard coral polyps. These plant cells also provide the coral's wide variety of colours.

Coral polyps grow in colonies, which means that each individual animal is attached to another, and then another. Food can be passed from one polyp to another through tubes connecting the polyps called coenosarcs. A colony can grow to be quite large. In the reef at Speyside is one of the largest brain corals known worldwide, over 10 feet in diameter! It contains many thousands of individual coral polyps all living together. A larger reef is formed by many coral colonies, often with many different kinds of coral. A reef may be hundreds of miles across, but it is still built by millions of tiny coral polyps.

A coral polyp is an invertebrate animal -- that is... an animal with no backbone. However, coral polyps do have skeletons, which they make with limestone. Thousands of these tiny skeletons combine to become the structure of a reef. 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 coral colonies and the reef.

The limestone is made of calcium carbonate. Calcium is extracted from the seawater by the polyps and combined with carbon, which is a by-product of their respiration, to produce this limestone. Corals can be either rock hard or soft. The combined skeletons of hard corals produce a rigid, boulder-like reef. Soft corals also have a skeleton -- actually tiny needle-like splinters called spicules, generally made out of calcium carbonate. The spicules are embedded in the colony to give it strength. But the space in between these spicules enables the colony to bend and sway with the currents.

Soft corals look like trees or bushes. There are many varieties of soft corals, including branching corals and gorgonians or sea whips. The brightly coloured sea fan is another type of soft gorgonian coral. It is easy to see how soft corals could be mistaken for plants. But just like hard corals, they are a collection of coral polyps. While soft corals also grow in colonies, they do not form reefs like hard coral.

Reef-building is a very slow process. Staghorn coral, for instance, grows at approximately one centimetre per year. It is estimated that present-day Buccoo Reef represents ten thousand years of coralline growth and reef formation. There are about 735 species of reef-building corals the world over of which 39 are known to exist in the waters around Tobago.

LIFE ON THE REEFS

Coral reefs are best known for their breath-taking variety of colourful marine life which they harbour. Tobago's reefs boast an array of brightly-coloured fish including, among others, parrot fish, wrasses, spot-fin and other butterfly fish, trumpet fish, toadfish and angel fish. Reef fish feed on smaller organisms found within the confines of reef waters and, in some instances, on the coral themselves. Fishes however, are not the only colourful components of the reef ecosystem.

Many corals are themselves brightly-coloured, ranging from red through orange to grayish-white. In addition, there are the flambouyant sea whips, fans, brittle stars, molluscs and different varieties of sea urchins (sea eggs), some of which are transparent, and of different shapes and sizes. To say nothing of the sea anemones and sponges and various algae (sea weeds) which abound in the coral reef habitat.

Many corals are themselves brightly-coloured, ranging from red through orange to grayish-white. In addition, there are the flambouyant sea whips, fans, brittle stars, molluscs and different varieties of sea urchins (sea eggs), some of which are transparent, and of different shapes and sizes. To say nothing of the sea anemones and sponges and various algae (sea weeds) which abound in the coral reef habitat.

THREATS TO THE REEFS

A wide variety of human activity has damaged almost all of the world's coral reefs in recent years. Human damage also weakens the reefs' ability to recover from natural disasters. Some experts predict that unless changes are made soon, most of the world's coral reefs will be dead in just 20 to 40 years from human causes. This would be a major catastrophe for life in the world's oceans, and for the human communities dependent upon the reefs for food, income, medicine, and coastal protection.

Ocean pollution poisons coral polyps. Pollution takes on many forms including oil slicks, pesticides and other chemicals, heavy metals, and garbage.
Fertilizer runoff and untreated sewage introduce added nutrients to coastal ecosystems. These elevated nutrient levels promote algae growth. Unfortunately, high concentrations of algae or solid sewage can overwhelm and smother the polyps. Under normal conditions, herbivores, fish and some invertebrates keep the algae population in check.

Deforestation degrades more than just land habitats. When tropical forests are cut down to clear land for agriculture, pasture, or homes, topsoil washes down rivers into coastal ecosystems. Soil that settles on reefs smothers coral polyps and blocks out the sunlight needed for corals to live.

Coastal development and dredging ravages reefs. This development includes building seaside homes, hotels, and harbours.

Besides fishes, fishermen harvest a variety of exotic seafood from the reef including conchs and lobsters.

Visitors to Buccoo Reef can view the reef through glass-bottomed boats. The practice of allowing tourists to leave the boats and walk over the reef has severely damaged large areas of the reef and this practices continues, even after Buccoo Reef was designated a restricted area in 1973, the reasoning being that those portions where people are allowed to walk are already damaged by previous walkers.

Coral reefs are among nature's greatest spectacles, producing some of the finest examples of natural architecture in the world. They form communities of startling complexity and are essential to the health of the world's oceans and to many human communities as well.

Will the world's coral reefs (including Tobago's) survive so that future generations can experience their stunning beauty and continue to benefit from their many gifts?

Scientists worldwide have recently issued both a warning about the dire state of coral reefs and a challenge to preserve them.

Their fate is now in the hands of all us alive on earth today.

Bibliography

The Coral Reef - Oasis In the Sea - Our Marine Heritage, Donna Spencer, Institute of Marine Affairs
Coral Reefs of Trinidad and Tobago - Institute of Marine Affairs

Rainforests of the Sea - Video, Oceanic Research Group
 
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