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CORAL ECOSYSTEMS Coral growth A coral polyp is an animal essentially consisting of a mouth surrounded by tentacles and a simple body cavity (Figure 24, 45). The polyp can be thought of as the building block of a colony. The physiological link between coral and alga, resulting in exchange of metabolic products, ensures that rapid coral skeleton building is dependent upon light. A coral colony begins when a single polyp grows by budding or cloning new and identical polyps. The colonial nature of corals means that the size and life span of individual polyps are not limiting factors of ultimate colony size and age. Theoretically this may be unlimited, but in practice some species tend to grow larger and live longer than others.
Many coral species can produce colonies with life spans of decades, but only a few are confirmed to be able to live for multiple centuries as intact colonies. Each polyp secretes a skeletal cup of calcium carbonate beneath the external veneer of living tissues, into which the polyps (of most species) can withdraw for protection. Growth involves the formation of new skeletal cups on top of the old ones, as well as the adjacent budding of new polyps.
At the other end of the range, massive Porites bommies, that can reach sizes in excess of 9 metres in height, may achieve radial colony increments as low as 8 mm per year. The majority of species, however, grow at intermediate rates, with many massive forms achieving radial increments of 1-2 cm per year and plate type and branching corals somewhat more.
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