How Corals Grow
Corals grow as calcite, secreted at the base of a polyp, which accumulates, usually in the form of a column. Growth proceeds according to a few simply defined rules, and these basic processes themselves may modulate the growth by diffusion-limited accretion (DLA), outlined in the text.
Many corals are autotrophic to varying degrees: they use photosynthesis by resident zooxanthellae as a source of energy to drive calcite deposition. Their growth is influenced not by one but by two potential energy gradients— gradients in nutrients brought in by flowing water and in light—having independent effects on growth. Montastera corals in shallow water, where light levels are high and the light is without a strong directional component, grow into a hemispherical shape. In deeper waters, where the direc tion of the incoming light is mostly from above, Montastera will form stout columnar branches, because the upper surfaces receive the most light and, hence, nutrients from zooxanthellae. Upward growth is amplified by the boundary layer effects described in the text. In shaded areas, or in deep water where light does not penetrate, the greatest input of nutrients will be at the edge of the coral, where the polyps face into the flow. Lateral growth of the coral into a horizontal plate will therefore be favored.
A few rudimentary growth models can explain a lot of the diversity of growth types among corals. One of the more fruitful mimics the growth of trees, which also grow from the tips like corals. The analogy between trees and corals is strengthened by the fact that some corals move nutrients between polyps through connections called coe-nosarcs, in just the same way that trees move nutrients between its parts through the phloem and xylem. Tree biologists have formulated four basic models for the growth a.
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