Light profiles beneath the tabular coral speciesAcropora cytherea and A. hyacinthus were examined at Rib and Broadhurst Reefs, central Great Barrier Reef, in November 1980. They show a strongly decreasing illumination gradient towards the central stem. Beneath tables at lOre deep, which receive about 7.0 mW cm -2 at solar noon, substrate irradiance falls to a minimum of about 5% ambient or to about 0.4 mW cm -2 . Thus, from previously reported compensation values, most sub-table substrate may receive above-compensation irradiance. Illumination beneath tables screened by foil is significantly lower. The coral communities beneath tables of these two species plus those ofA. subulata andA. clathrata support an average 26 colonies m -2 , at a cover of 40%, nearly identical to adjacent, unshaded quadrats. Species richness is likewise very similar. Species richness and colony density beneath the tables increase inwards from the perimeter of the shaded areas, decreasing only near the central stems. Dark adaptation and reduced competitive and grazing pressure are suggested explanations for the latter. It is concluded that shading does not provide a significant competitive advantage for Acropora spp. tables at 10 m deep. competitive advantage over other corals (e.g. Connell, 1973;Porter, 1976), thereby reducing photosynthesis or growth of the latter. However the degree of shading caused by the Acropora table canopies has never been measured and, as pointed out by Woodin and Jackson (1979), its importance has not been assessed.
Several coral species grow over depth ranges of 50 m or more, or over ranges of light intensity which span more than 2 orders of magnitude (Wells, 1957b;Barnes and Taylor, 1973;Sheppard, 1980Sheppard, , 1981)). Although many species live in more restricted ranges of illumination, this may not be a limiting effect of light per se (Falkowski and Dubinsky, 1981). If shading beneath tabular Acropora species is important ecologically or to the survival of the Acropora colony, a changed community beneath it might be expected. Earlier observations on reefs at 10 m deep however, showed as high a diversity of corals beneath a canopy of A. hyacinthus as in areas lacking the overshadowing species (Sheppard, 1980), which suggested that the effect of shading was not marked.
The following examines the light conditions and the coral species composition beneath table corals. On the basis of these observations, the competitive value of shading is assessed.