The Great Barrier Reef
Nearly all corals live in symbiotic relationships with Zooxanthellae algae, which rely on the coral for protection and for access to the light necessary for photosynthesis. In return, the algae provide the coral with food and food also give their distinctive colors. Other marine invertebrates and fishes live in the coral reef community, which provides shelter, food, and a habitat for nurseries.
Coral secrete a hard calcium carbonate skeleton, which forms the substrate on which they live and which becomes a coral reef as it builds up over time. The calcium carbonate skeleton also protects the corals from predators. Scientists have categorized coral reef into three main types: fringing reefs, barrier reefs, and atolls. Their geographic range includes both tropical and temperate climatic zones, though they are most abundant in warm, shallow water. The geological range of coral reefs is from the Middle Ordovician to modern times.
Corals play an essential economic role for many coastal human communities, as they provide a habitat for fishers, physical protection from erosion, a source of marketable goods, and a destination for tourists. Unfortunately, coral reefs globally are showing signs of ecological degradation from pollutant runoffs of chemicals and human sewage, increasing sedimentation and eutrophication, damage from ships and divers, and increasing sea temperatures and ultraviolet radiation exposure. Coral reef ecosystem health is an important indicator of the global environmental condition, as they are highly sensitive to changes in water chemistry and temperature.
The largest reef in the world measuring 2011 km in length and 72 km across at its widest point is the Great Barrier Reef. It stretches along the coast of Queensland in Australia. It is a true wonderland of color and beauty. Of the many hundreds of varieties of coral growing on the reef, the Staghorn (antler type) is one of the most common.
The Great Barrier Reef
Is a natural barrier made of the bodies of living and dead coral. It is normally just below the surface of the water. It is made of a white part containing the bodies of zillions of polyps which have died hundreds years ago and a colorful part that is the living part of the coral reef. It is made up of living polyps.
As the world’s largest coral reef ecosystem the Great Barrier Reef is home to approximately:
1.500 species of fish
400 different types of corals
4.000 species of mollusks
500 species of seaweed
215 species of birds
16 species of sea snake
6 species of sea turtle
Whales visit during the winter.
The coral itself is food for many species. The butterfly fish eats individual coral polyps, while the Crown-of-thorns starfish eats entire coral colonies. Algae growing on the coral is eaten by many major herbivores, who scrape the algae from the coral before ingesting it. Some creatures sift through bottom sands for invertebrates. Stingrays crush the hard shells of mollusks to get to the soft bodies inside. Various species are scavengers, and feed on detritus. There are numerous predator- prey relationships and complex food webs.
Threats such as pollution from oil spills, land runoff and anchors from boats all cause damage to coral reefs. Increases in nutrients allow bacteria levels to rise, and the bacteria reduce oxygen levels through respiration. People damage the reefs by walking on them, dragging diving gear over them, breaking them and taking them as souvenirs and by over fishing. Natural outbreaks of some organism can also cause destruction in coral reef ecosystems. The spiky creature is the Crown-of-thorns starfish, which inverts its stomach out of its body to cover and digest the coral, leaving only the skeleton behind.
It is not yet known whether outbreaks of this creature are natural or related to human activities, although it has been suggested that over collecting of its natural predator (the giant triton) by humans has been a major influence. It is known however that during feeding these creatures releases certain chemicals, which tend to attract others. One old (and now unused) method of attempting to kill the starfish included cutting them up into small pieces, after which they were simply thrown back into the water. Many starfishes are able to regenerate from parts of themselves, so you can imagine that this method was not used for very long.
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