While all corals contribute in some way to the ecology of the reef, certain families and/or genera dominate by making a major contribution to the reef framework, coral cover, species diversity, age structure, viability, coral/organism interactions and many other aspects of the overall operation of the reef. These groups tend to be the focus of the most research and to provide much of the information relevant to understanding and managing coral reefs in the face of a multiplicity of demands and threats to their persistence as healthy ecosystems. Presented below is a discussion of the six major families represented on the GBR. See Box 20.2 for a listing of all families and genera.
(1) Family Acroporidae (Fig. 20.5A-F)
This family includes the two most diverse genera of living zooxanthellate corals, Acropora and Montipora and also the genus Isopora, with just a handful of species. These genera often dominate the outer reef flat and the reef front, down to around 15 m, after which more species from the less prominent families can be found. Acropora (known as the staghorn coral genus), has a unique branching mode in which one type of cor-allite (the axial) forms the centre of the branch and buds off many more of a second kind (the radials) at its tip as it extends. This has led to numerous colony shape possibilities. Additionally, growth of the colony may be symmetrical and determinate (slowing down as a determined shape is reached) or asymmetrical and indeterminate, the colony growing to fill available space on the reef. As many as 40 species of this genus may occur in 500 m2.
Acropora has over 120 species worldwide and over 70 on the GBR. Probably because of its great abundance, Acropora suffers the most from storm and cyclone damage, coral diseases, grazing by predators such as the crown-of-thorns starfish Acanthaster planci and aggregations of Drupella gastropods, and coral bleaching due to elevated sea-surface temperatures. Acropora, Isopora and Montipora are usually abundant among recruiting corals in experimental and natural situations and some species of Acropora also recruit asexually by growth of fragments after storm damage. Isopora is unique in the family in that its species are brooders, whereas all other Acroporidae are broadcast spawners.
(2) Family Faviidae (includes many of the 'brain corals') (Fig. 20.5G-Í)
In this family the genera are massive, encrusting, or, less commonly, branching, and corallites have numerous septa, well-developed columellae and walls formed by simple fan systems of calcium carbonate crystals (trabeculae). Faviids, the largest family in terms of the number of genera, contribute to the reef by their densely constructed skeletons and their wide-ranging environmental tolerances. Genera are distinguished by the type of polyp division (extra- or intra- tentacular) and the arrangement of corallites (either separated or in contact, or combined in valleys to form the meandroid form that leads to the name 'brain corals'). Species distinctions within genera are based on dimensions of the corallites, features of the septa, costae, coenosteum and columella and number, shape, ornamentation and elevation of the septa above the corallite.
Faviids can be seen in most reef habitats, except perhaps very sandy locations. They are common on the intertidal reef flat and submerged shallow shoals, and also on fringing reefs along the coastline and continental islands. They can be the dominant coral on fringing reefs, such as the reefs of Moreton Bay at the port of Brisbane and on the exposed reef flats on the western side of Magnetic Island, off Townsville. Faviids form a major component of the coral mass spawning events in eastern
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