ReefDiary

The world's first coral reefs occurred about 500 million years ago, and the first close relatives of modern corals developed in southern Europe about 230 million years ago. By comparison, the Great Barrier Reef is relatively young at just 500,000 years old. The current reef's structure is much younger at less than around 8,000 years old.Most modern reefs have formed on hard surfaces in the ocean, such as a base of an old reef that died during a period when sea level was lower, or the edge of a rocky island. Depending on how they start out, several types of reefs can form. Some coral reefs form in the deep ocean and are called atolls. The theories on how coral reefs form were first put forward by Charles Darwin (of The Origin of Species fame) who proposed that atolls form around the edges of high volcanic islands that gradually submerge beneath the sea with changes in sea level or subsidence of the land. Thus an atoll starts life as a fringing reef, then becomes more of a ring growing on the shrinking land-mass, until the land disappears and just the coral circle remains. In some cases, the coral growth is unable to keep pace with the sinking island, and sunken dead reefs have been found. Class: Anthozoa [includes corals, anemones and sea pens]Habitat: Coral reefs are found in shallow water, ranging to depths of 60 m. Some species prefer either cooler temperate water while others are found along tropical reefs, such as the Great Barrier Reef, with waters ranging in temperatures from 18 - 33 °C. Living in colonies: They generally occur in large numbers as colonies of individual polyps linked by tissue. Resources, such as food, are then shared amongst the individuals in the colony. Coral Size: Individual polyps range from 3 - 56 mm in diameter or height; while colony size varies from 75 mm -1500 mm (1.5 m) in width, height or length.
Some corals have a mutualistic relationship with algae called zooxanthellae. A mutualistic relationship is one where both parties benefit from their partnership. The algae use sunlight and the polyp’s waste products to make oxygen and food. These substances leak into the surrounding tissues of the polyp and can provide up to 98% of the polyp’s dietary requirements. These corals are found in shallow water, as they require sunlight to survive. They are generally fawn, brown or green in colour, due to the yellow-brown colour of the zooxanthellae. Stinging cells: All Cnidarians have characteristic stinging cells called nematocysts in the tentacles and body wall. Each nematocyst cell contains a coiled thread under pressure, which is ejected from the cell when triggered by touch. These stinging cells are used for catching prey and for defense, some having barbed ends connected to poison sacs, while others are sticky.

source:http://www.barrierreefaustralia.com/the-great-barrier-reef/coralfacts.htm

posted by TheReefsLife's @ 12:21 AM