Coral skeletons are made of aragonite, a type of calcium carbonate. Using aragonite crystals, they build a frame that will thicken and create strong skeletons. Bundling of aragonite crystals builds the sturdy skeleton that is able to endure currents, storms, marine life, etc. Major components of a coral polyp's body include the mouth and tentacles (nematocysts or cnidae). The sac-like body of the coral polyp is very delicate and the skeleton is able to protect the vital organs. Coral skeletons also have a lot of influence on colony size, branch size, growth form, etc based on the strength of the skeleton. 

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Distribution of Magnesium in Coral Skeleton

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A species of reef-building coral, Pavona clavus, has shown that magnesium and strontium are distributed differently. Magnesium's distribution within the coral skeleton has been found to be correlated with the organization of aragonite fibers. Aragonite fibers that are at the centers of calcification have been found to have up to ten times the concentration of magnesium showing that there may be a biological process that purposely controls and regulate components of skeletal crystal. Coral skeleton growth that was once considered a physicochemical process, has been questioned to now think the process can be biological. Magnesium has been studied to be a vital nutrient for the formation of calcium carbonate phases, calcification stages. 

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Skeletal Strength

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A study done by John A. Chamberlian Jr. proved that the strength of three common Caribbean corals was more than concrete. The strength of coral skeleton varies inversely with skeletal porosity meaning that lower porosity means enhanced strength. This is crucial to corals in their current environment since their marine environment has changed significantly because of the change in seawater chemistry, making corals more stressed. This is a valuable adaptation for corals to have as seawater chemistry continues to alter the coral's environments, corals must maintain strong skeletons in order to survive and adapt to the changes. During this research, skeletons of corals were compared in different environments: highly stressed versus moderately stressed. Corals not living in challenging environments, will have higher porosity and therefore, less strength compared to corals living in highly stressed environments. 

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Skeletal architecture is important in determining the strength of the skeleton. Two factors that are important regarding its architecture include polyp orientation and skeletal porosity. For most corals, polyps grow vertically that make a grain-like structure similar to bone and wood. It is advantageous to have this type of structure in order to resist compressive stresses and tension. Skeletal porosity refers to pore volume and total skeletal volume. In the coral skeletal strength, there is an inverse relationship between porosity and strength. Within the skeleton, more porous structures tended to have less strength. Porosity modification could be possible for corals, possibly within a single colony, where corals could thicken walls or structural parts where stress may be higher.

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It is known that among other calcifiers, it is among the weakest of ceramic skeletal animals. However, it is not completely necessary for corals to be extremely strong. Although exposed to strong waves, polyp-eating predators, storms, etc., they are able to minimize mechanical degradation. By selecting habitats that have calmer waters or have barriers that are able to protect corals for strong currents, it would not be necessary for those corals to be strong since extreme habitats can be avoided. However as mentioned with porosity modification, corals are able to strengthen their skeletons so why don't they? If corals are living in an environment that experiences an occasional storm, it is more costly for the coral to expend more energy over time to build a stronger skeleton versus just going through reproduction.

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