Spatial pattern in the abundance and structure of mollusc populations in the soft sediments of a coral reef lagoon

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Abstract
A hierarchical sampling programme, carried out during October 1985, encompassed 4 spatial scales: broad habitat zones of different depth and sediment 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.sbd.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, intermediate-sediment 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, species 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.