Classification And Identification

The classificatory scheme for corals was developed by reference to the elements of the skeleton. Understanding this requires knowledge of the parts of the skeleton underlying various polyp features and the way these vary from family to family (Fig. 20.2). Paradoxically, the exoskeleton of a coral appears to an observer to be inside the polyp, but actually it is underneath. The remarkable process of skeletal production leads to a structure that supports almost the entire three-dimensional form of the polyp as well as the connecting tissue and continuation of the body cavity (coenenchyme) between polyps. This creates a colony architecture that allows species to be identified both in life and after death, without reference (in most cases) to the tissue structure of the polyps themselves. After death, the skeleton persists, although it may be subject to erosion by other reef organisms, and as a

Figure 20.2 Structures and terms used in the identification of reef corals. A-F, terms relating to overall colony shape and polyp arrangements in corals; G-L, terms relating to forms of corallite arrangement in corals. A, massive; B, encrusting; C, plating; D, solitary free-living; E, branching arborescent; F, branching tabular; G, plocoid; H, ceriod; I, meandroid; J, phaceloid; K, hydnophoroid; L, plocoid with coenosteum features. (Photos: P. Muir.)

Figure 20.2 Structures and terms used in the identification of reef corals. A-F, terms relating to overall colony shape and polyp arrangements in corals; G-L, terms relating to forms of corallite arrangement in corals. A, massive; B, encrusting; C, plating; D, solitary free-living; E, branching arborescent; F, branching tabular; G, plocoid; H, ceriod; I, meandroid; J, phaceloid; K, hydnophoroid; L, plocoid with coenosteum features. (Photos: P. Muir.)

consequence, corals have perhaps the best fossil record A revolution in higher level classification of corals is of all animals. Much can be learnt about the history, being led by molecular biology (genetics), which is also geography, and future of living corals by reference to being used to study boundaries and relationships fossils. between species, population level genetic diversity and even the identification of individuals and species. Differences in skeletal morphology, often microscopic and previously missed in the skeletal classification scheme, are being found to validate many of the distinctions found by molecular analyses and these will lead to some name changes in the classificatory scheme over coming years.

Because of the major role of the skeleton in the identification of corals, a vast terminology has developed to cover the various skeletal components and some of these parallel the tissue components. The skeleton formed by the colony or part of the colony is the corallum and that formed by the polyp is a corallite; this has dividing elements called septa (singular septum) that support the mesenteries and often a central element, the columella, which is also part of the support system of the polyp. Septa may extend outside the polyp as costae (singular costa). Interconnecting areas of skeleton are referred to as the coenosteum. All of these elements may be ornamented in various ways and even the basic process by which they are laid down affects their appearance and forms the basis of the classification of families. Supporting structures below and between the polyps are dissepiments and these represent layers of skeleton laid down as the polyps and connecting tissues move upwards in the colony as they (and the colony) grow. A set of terms has also been developed to describe the layout of corallites within the corallum and these are used so often in descriptions of corals that it is very useful to master the basics of them.

More than 450 species of corals have been recorded from the waters of the GBR, eastern coastal Australia, and continental and Coral Sea islands. This chapter concentrates on the field appearance of corals and their identification in situ on the reef. When accurate identification to species level is required, especially for those corals with microscopic polyps, further microscopic identification may be necessary. Guidance for this can now be found in numerous publications, and coral collections for consultation are held in universities and museums; coral identification workshops and coral identification services can be found at some of these.

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