Patterns within assemblages of the coral genus Acropora on the Great Barrier Reef, Queensland

Abstract

Problems associated with taxonomic and field study of the coral genus Acropora were explored by means of censuses carried out on a reef flat and an outer flat - reef slope. A total of 138 × 10m transects parallel to the reef front were measured for cover, abundance and qualitative aspects of the Acropora colonies present.The corals were classed according to species, colony shape and radial coral lite shape, and the distribution of these different categories in the study areas analysed, using numerical methods based on information theory, as well as simpler forms of sorting. Some aspects of skeletal morphology were examined in detail.On final assessment, it was seen that populations had been sampled for 36 species of Acropora on the outer flat - reef slope, and 13 species on the reef flat. With overlaps, a total of 38 species was involved.It was found that the broad physical zones of reef flat (with mainly horizontal component) and reef slope (with mainly vertical component) were matched by species assemblages with only limited overlap. The reef crest region, where the transition from one to the other physical component occurs, had a mixed assemblage but some special characteristics of its own.The reef slope had greater cover, abundance, colony size and species numbers (when only Acropora was considered) than the reef flat, except the base level of the reef (just deeper than 10m), where sudden attenuation of all these characters occurred. Other features (colony shape, proportional radial-axial coral lite numbers) appear to alter suddenly from the beginning of this below buttress region. This change might indicate the changeover from optimal to light intensity, at least for Acropora.Some significant differences were seen between two compared portions of the outer reef: one steeper and more exposed than the other. Here a gentler slope showed significantly higher cover and abundance in the region than a steep slope. Three species forming large flat table-plates were more common on the outer flat - reef crest - upper area of the gentle, less exposed area, giving larger colony size and lower diversity, particularly just the crest.Colony shapes, on full appraisal, were found to be a composite of outline profile, branching pattern, and conformation to a degree of in growth. In outline a flat upper surface was seen to be a recurrent feature with different implications in different zones. When branching patterns were regarded as having primary and aspects, the secondary mode was found to be virtually absent on the reef flat, whereas both modes occurred on the reef slope. The somewhat arbitrary or ill-defined concept of determinance was examined, and it appears that the species relatively less determinate (or less limited) in growth are capable of great expansion and flexibility of shape, but cannot form the flat plate shape which appears to be necessary in the sub-optimal light regions of the reef slope.Radial corallite shapes were seen to have much descriptive overlap, and most species had variations which could not be accounted for by distribution alone. In some species distributed over a depth gradient, the colonies towards the bottom of the gradient showed thinner branches with more scattered radial corallites. It appeared likely that the character of the coenosteum had some influence on the observer's appreciation of radial corallite differences amongst species. The shape of the radial corallites in many species varied in an apparently polymorphic manner amongst the samples taken.A preliminary study of coenosteal structure with scanning electron microscopy indicated that the structure of coenosteal spines might yield concrete evidence for species groupings in Acropora.In summary, it can be stated that Acropora assemblages (sensu Rosen 1975) were sampled and found to be physically accommodated on the outer flat and region; biologically-physically accommodated in the crest region, and biologically accommodated in the region, to a greater extent when the slope is gentle. The reef flat assemblage before the outer flat is half Acropora, half other genera, and not so characteristically an Acropora assemblage. The area was judged to be suboptimal for Acropora, and it is predicted that the colonies here may not reproduce.