Observations of colony fission following ledge formation in massive reef corals of the genus Porites

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Abstract
Massive colonies of the coral genus Porites (Scleractinia) are among the most important reef-builders of the Indo-West Pacific. Colony fission is an important means of asexual reproduction in these species, resulting in the production of genetic clones or ramets. Fission occurs either by disturbance, or during undisturbed growth, as a consequence of formation of ledges around lower portions of large colonies. Outward growth of ledges shades and kills the tissues directly beneath, causing tissues nearer the base of the colony to become isolated from the parent. Of 47 colonies > 100 cm in diam. at North Brook Island Reef, Great Barrier Reef (GBR), 26 ledged colonies supported 148 ramets, of which 106 were established on 5 of the largest corals in the site (colony diam. > 350 cm). Ramets episodically become separated from the parent colony, a process facilitated by the relatively light and weak skeleton of Porites and enhanced by bioerosion and wave action. Ledges form from the horizontal expansion of lobes, probably in response to reduction in illumination caused by the upward growth of colonies. While 6 species of Porites on the GBR are known to exhibit this morphology - P. lutea, P. lobata, P. solida, P. australiensis, P. myrmidonensis and P. mayeri - not all large colonies from similar biotopes develop ledges. Whether this morphological variation is attributable purely to environmental effects on the phenotype, or also to genotypic differences within the massive Porites specie assemblage, is unknown at present.