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"Teaching people about the world in which we live."

FACT SHEET ON CORAL OF THE GREAT BARRIER REEF

Instructions:  While you read this look for answers to your questions. Write down notes about the answers you find on scrap paper.  Be sure that you can summarize the main points of information about coral reefs, paying special attention to the vocabulary words provided on the vocabulary sheet.

The Great Barrier Reef, located off of the eastern coast of Australia, is the longest coral reef in the world. It is roughly 1,240 miles long, stretching from the Tropic of Capricorn, north to southern New Guinea. It is farthest from the Australian coast at the southern end (about 100 miles from shore) and closest at the northern end (about 7 miles from shore). The Great Barrier Reef is home to 1500 species of tropical fish and 400 finds of hard and soft coral. It is an ecosystem that requires very specific conditions - the water must be warm (between 69-84 F) and relatively shallow (to depths of 60-100 ft). It is a fragile environment in which even slight changes can affect the balance and health of the entire system.

Coral reefs exist in shallow waters because of their dependence on photosynthesis for food production. Photosynthesis is the chemical process by which plants capture sunlight and convert it, along with water and carbon dioxide, into energy. These plants release oxygen and other organic materials as waste products of photosynthesis; in turn, these by-products are what animals use to produce their own food. In shallow water, more sunlight is available for photosynthesis. Tropical waters are generally poor in nutrients because there isn't much mixing of deep water and surface water. Nutrients from decomposing organisms that would otherwise be available for food sink to the ocean floor when those organisms die and remain there, instead of being recycled into the water closer to the surface. That is why reefs are so dependent on photosynthesis for energy.

The coral reef itself is actually a colony made up of many tiny organisms called coral polyps that are joined together by their outer skeletons. Each polyp resembles a tiny, soft pouch (some as small as a pin-head) with a mouth at the top that is surrounded by several stinging tentacles. The tentacles help the polyp collect some of its food at night by stinging plankton, but the most essential part of the polyp's food source comes from a separate organism living inside of it. This inner organism is a microscopic, unicelled (one-celled) algae called zooxanthellae. Zooxanthellae can be thought of as the "building block" of the coral reef. It uses photosynthesis to produce its food, and the oxygen and nutrients (such as sugars and amino acids) it produces as waste products can be used by the polyp for its own food. In turn, the waste products of coral polyps include materials needed by the zooxanthellae to continue the process of photosynthesis.

This kind of mutually beneficial relationship between the coral polyp and zooxanthellae is a form of symbiosis. A symbiotic relationship is one between two separate organisms in which they live together to the benefit of at least one of the organisms. There are several different kinds of symbiotic relationships in nature. Parasites, such as leeches, have a symbiotic relationship with their host; they take their nourishment from the host, but the host is harmed in the process and often dies. The symobiotic relationship between coral polyps and zooxanthellae is not parastic, though; it is a different form of symbiosis, called commensalism. Both organisms benefit from living together, without interfering with each other.

There are other benefits to this symbiotic relationship besides the exhange of food nutrients mentioned above. While the body of the coral polyp is soft, it protects itself and joins together with other polyps by growing its own hard, outer skeleton. This skeleton is composed largely of a substance called calcium carbonate, similar to the material that our own teeth and bones are made of. The presence of zooxanthellae within the polyp allows more calcium carbonate to be available to the polyp and increases the rate at which the skeleton can be built. The zooxanthellae benefits from living within the coral polyp because it gains an extra layer of protection by the presence of the outer skeleton, and it also has an opportunity for greater exposure to the sunlight it needs for photosynthesis.

Why is the coral of the Great Barrier Reef important?

There are a great number of benefits provided by the Great Barrier Reef. First of all, the reef serves as a protective habitat for many other forms of marine life. The outer edge of the reef receives the full force of breaking waves, protecting the inner Australian shoreline. This sanctuary between the shoreline and the inner edge of the reef acts as a kind of nursery for growing fish and other marine life. In addition, the calcium carbonate in coral skeletons helps maintain the pH balance in the ocean needed to sustain life. Interestingly, medicine has also made important discoveries within coral reefs. Some hard coral is being experimented with as a possible bone replacement, and treatments for diseases such as cancer, AIDS, asthma, arthritis, and various infections have been developed from organisms in coral reefs.

There are several threats to the health of coral reef environments. One the most troubling problems is that of coral bleaching. For reasons that are still being studied, coral polyps sometimes eject the zooxanthellae living inside them (or the zooxanthellae ejects itself). When this happens, the coral loses its primary food source and basically dies of starvation and turns white. There are scientists tracking "hot spots" of coral bleaching around the world.

Another threat to coral reefs comes from predators such as crown-of-thorns starfish, which feed on the soft coral polyps. Usually, the number of crown-of-thorns starfish remains in balance, but occasionally, "outbreaks" occur in which the population becomes unbalanced and threatens to destroy areas of the reef at a rate faster than that at which it can rebuild itself. In response to these outbreaks, there have been times when divers have actually been encouraged to remove crown-of-thorns starfish from the coral reef; this must be done very carefully, though, because crown-of-thorns starfish are covered with poisonous spines.

There are man-made dangers to the Great Barrier Reef as well. Damage can be done to coral through pollution from sewage, oil spills, and fertilizers. When boats run aground or drop heavy anchors on reefs, the protective coral skeletons are broken, exposing polyps to damage or death. Some fishing methods can also harm coral; these include using bleach or other chemicals to catch lobster or fish and overfishing an area. Explosive mining is another destructive force. Additionally, if you are ever able to go diving in a coral reef, do not touch or stand on the coral; this can also cause damage.

The Great Barrier Reef is home to an astounding variety of marine life. From it, we can gain a greater understanding and appreciation for the marine ecology and for the delicate balance that maintains it.

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