The structure and composition of the infauna of the coral P. damicornis in the Capricorn Group, Great Barrier Reef, is examined. A total of 40 coral heads were sampled from four localities on two reefs and 951 individuals of 101 species were extracted. This is five times the species collected from P. damicornis in previous studies. A comprehensive species list is provided. Infauna was dominated by only a few species, 20 species accounting for 78% of individuals and 36 species recorded only as single individuals. Crustacea was the dominant taxon present, comprising 72% of species and 86% of individuals. Number of species and number of individuals show significant regressions with size of coral heads. Differences in physical environment at the four sites did not affect species diversity within coral heads. However, several common species only occurred within particular habitats. Species diversity is discussed briefly in relation to the behaviour of resident symbionts, random pelagic settlements of larvae, the size of target corals and habitat heterogeneity in coral heads.

ABSTRACT. A hierarchical sampling programme, carried out during October 1985, encompassed 4 spatial scales: broad habitat zones of different depth and sebment composition; random 1 ha locations within zones; random 25 m2 sites within locations; and random 0.01 m2 air-lifted cores from each site. Of 69 species collected, we describe the patterns of distribution of the 10 most common. Spatial differences in their abundances were observed at all scales, but most of the variation (50 to 85 %) occurred at the broadest scale -between zones. Zones of peak abundance differed among species, some being most abundant in deep, fine-sediment habitats, e.g. Tellina cf. myaformis; some in deep, intermediatesediment habitats, e.g. Exotica cf. virgulata and Pupa nitidula; others in shallow, coarse-sediments to the north, e.g. Tellina obtusalis, Umbonium guamensis; and still others in the shallow sediments to the south, e.g. Rhinoclavis aspera. In terms of population structure, tellinid bivalves reached greatest sizes in the coarse shallow zones. At the broadest scale, zones did not appear to represent homogeneous assemblages, due largely to a variable response of species to the environment at a location scale. Overall, specles richness tended to increase from fine to coarse sediment, except at the very shallow coarse sediments in the southern zone, where low numbers of species were found.

On the basis of repeated censuses of unmanipulated assemblages of fish on 20 lagoonal patch reefs. Sale & Douglas (1984) proposed a simple random placement model to account for the structure of such reef fish assemblages. The present paper uses their data augmented by additional censuses and a series of simulation models to explore the adequacy of their proposal. The distributions of a small proportion (about 8 species) of the 143 species recorded on the reefs, but including some of the most abundant species, could not be explained by a process of random colonization. These species vary greatly in abundance among censuses, and tend consistently to prefer certain reefs. The remaining species, while clearly possessing some microhabitat requirements, and perhaps responding to each other's presence, show distribution across patch reefs which are satisfactorily explained by a model of random placement of juveniles from a pool of potential colonists.

Data on the distribution of 25 common species of fish over 20 patch reefs located in a coral reef lagoon (Great Barrier Reef) were collected by 20 visual censuses over 10 yr. Associations among the 25 species were examined by cluster analysis. The overall pattern of association changed substantially from census to census. So too &d the relationships among small subsets of the 25 species which were tightly associated at one or more censuses. The degree of association among groups of species was compared with that expected under a null model of non-interactive recruitment of fish to patch reefs. Species in many apparently closely associated small groups were less strongly associated than expected under the null model, suggesting that species react differently to the habitats offered by the patch reefs, or respond negatively to each other's presence. Results are discussed in the light of other information on the structure of these assemblages of fish.

Large coral reef fish were experimentally excluded from enclosed plots for 2 yr to examine their effect on the dynamics of soft sediment mollusc populations from areas in One Tree lagoon (Great Barrier Reef). Three teleost fish which feed on benthic molluscs. Lethrinus nebulosus, Diagramrna pictum and Pseudocaranx dentex, were common in the vicinity of the cages. Surveys of feeding scars in the sand indicated similar use of cage control and open control plots and effective exclusion by cages. The densities of 10 common species of prey were variable between locations and among times. Only 2 species exhibited an effect attributable to feeding by fish, and this was at one location only. The effect size was small relative to the spatial and temporal variation in numbers. The power of the test was sufficient to detect effects of fish on most species, had they occurred. A number of the molluscs exhibited annual cycles in abundance, with summer peaks due to an influx of juveniles but almost total loss of this cohort in winter. There was no evidence that predation altered the size-structure of these populations. While predation by fish is clearly intense, it does not have significant effects on the demography of these molluscs. The results cast doubt on the generality of the claim that predation is an important structuring agent in tropical communities.