Growth and growth form of the massive coral, Porites

Darke, Wendy Marilyn (1991) Growth and growth form of the massive coral, Porites. PhD thesis, James Cook University.

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Abstract

Massive Porites colonies develop a bumpy growth surface as they increase in size. Development of a bumpy growth surface occurs when skeletal growth no longer provides the necessary increase in surface area to accommodate tissue growth. A massive Porites colony becomes bumpy when it reaches a critical size determined by the ratio of its tissue growth to its skeletal growth. This ratio also determines the degree of bumpiness which develops at the growth surface. X-radiographs of skeletal slices cut from the vertical growth axis of massive Porites colonies display annual density banding and skeletal architecture associated with corallites, that is, skeleton deposited by individual polyps. Density bands outline former positions of the growth surface. Examination of X-radiographs of Porites shows that new corallites are initiated on, or towards, the summit of bumps, whilst older corallites are compressed and ultimately occluded at the bottom of valleys formed between bumps. X-radiographs show that it takes 4 to 7 years from the formation of a corallite to its occlusion. Polyps on the growth surface of a bumpy Porites colony must, therefore, be continually lost. All polyps are lost and replaced during a 4 to 7 year period. Consequently, tissue covering the growth surface of a massive Porites colony can be no older than 7 years, even though the colony may have been growing for several centuries. Computer models designed to simulate growth of a massive Porites colony indicated that the growth form displayed by a Porites colony is determined by the ratio of tissue growth to skeletal growth. Models having a relatively faster tissue growth compared with skeletal growth developed a bumpy surface sooner, and the amount of bumpiness developed was greater, than for models having a relatively slower tissue growth compared with skeletal growth. Predictions from computer models accorded with observations and measurements made on actual colonies and on X-radiographs of skeletal slices cut from colonies. Thus, the ratio of tissue growth to skeletal growth determines important aspects of the growth form displayed by massive Porites colonies. The ratio of tissue growth to skeletal growth was shown to significantly affect the rate of polyp loss and replacement in Porites colonies. The longevity of polyps is less in Porites colonies displaying a well developed bumpy growth surface than in colonies displaying a smoother growth surface. Hence, the age of polyps, and therefore the tissue, covering a bumpy growth surface is less than polyps and tissue covering a smooth growth surface. Skeletal surface area in massive Porites colonies was shown to be a useful indicator of tissue biomass. Measurements of change in surface area of Porites colonies with increasing size show that the rate of tissue growth must decrease as the colony grows. Development of a bumpy growth surface alleviates this geometric restriction for only months to a couple of years. Development of a bumpy growth surface is an indication that tissue growth is becoming constrained by skeletal growth. Once a colony becomes bumpy, the tissue growth is almost totally constrained by the rate by skeletal extension. Significant differences in growth and growth form characterised massive Porites colonies collected from different reef environments. Measurements made on the colonies suggested that differences in environmental conditions probably altered the ratio of tissue growth to skeletal growth and caused the colonies to grow in different ways. Differences in growth were reflected in the resulting growth form. Information about relative rates of tissue and skeletal growth within a massive Porites colony gained from observations and measurements of the growth form can be used to provide further information about coral growth and details of environmental conditions obtaining during growth.

Item ID: 24102
Item Type: Thesis (PhD)
Keywords: corals; growth; Myrmidon Reef; Pandora Reef; Porites; Rib Reef
Date Deposited: 11 Dec 2012 07:27
FoR Codes: 06 BIOLOGICAL SCIENCES > 0608 Zoology > 060806 Animal Physiological Ecology @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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