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Stony Coral
Scleractinia, also called Stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian. Much of the framework of coral reefs is formed by scleractinians.
There are two groups of Scleractinia:
* colonial corals found in clear, shallow tropical waters; they are the world's primary reef-builders.
* solitary corals are found in all regions of the oceans and do not build reefs. Some live in temperate, polar waters, or below the photic zone down to 6000 meters.
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As mentioned above, Scleractinians may be solitary or compound. The most common forms include conical and horn-shaped scleractinians. In a colonial Scleractinia, the repeated asexual division by the polyps causes the corallites to be interconnected, thus forming the colonies. There are also cases in which the adjacent colonies of the same species form a single colony by fusing.
The rigid scleractinian skeleton, which lies external to the polyps that make it, is composed of calcium carbonate in the crystal form aragonite. The skeleton of an individual scleractinian polyp is known as a corallite. Each of its radially-aligned elements, termed septa, lies in the endocoel flanked by the members of a mesenterial pair. The skeleton originates as a thin basal plate from which the septa arise vertically. The structure of both simple and compound scleractinians is light and porous, rather than solid as in the Rugosa.
Septa are secreted by the mesenteries and are therefore added in the same order as the mesenteries. As a result, septa of different ages are adjacent to one another, and the symmetry of the scleractinian skeleton is radial or biradial. This pattern of septal insertion is termed "cyclic" by paleontologists. By contrast, in some fossil corals, adjacent septa lie in order of increasing age, a pattern that is termed serial and that produces a bilateral symmetry. Scleractinians are distinguished from the Rugosa also by their pattern of septal insertion. They secrete an aragonitic exoskeleton in which the septa are inserted between the mesenteries in multiples of six.
In scleractinians, there are two main secondary structures:
* Stereome is an adherent layer of secondary tissue, which covers the septal surface. It consists of transverse bundles of aragonitic needles and protects the scleractinians. However, its function can be nullified by the thickening of the septa itself.
* Coenosteum is a perforated complex tissue that separates individual corallites in a compound scleractinians.
At the beginning of Scleractinia's development four groups with different microstructure can distinguished. These are:
* Pachytecal: Corals having very thick wall and rudimentary septa. This is the group which probably originated from Rugosa corals.
* Thick Trabecular: Corals with septa built from thick structures, resembling little beams, called trabecules.
* Minitrabecular: Corals with septa built from thin trabecules.
* Fascilcular or non-trabecular: Corals with septa not built from trabecules, but from columns being bunches of aragonite fibres.
* There are two main controls on the form of a scleractinian colony. One is the mode of budding and the other is the relative growth rate. There are two types of budding: intratentacular and extratentacular. In an intratentacular budding, polyps are divided by simple fission across the stomodaeum, and each bud retains part of the original stomodaeum and regenerates the rest. Extratentacular budding takes place outside the tentacular ring of the parent. These daughter buds do not share any part in the functions within the parent scleractinians as do the products of intratentacular budding.
Information from Wikepedia - http://en.wikipedia.org/wiki/Stony_coral
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dead Stony Coral (Diploastrea heliopora) - photograph by Quinton Marais
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