Distribution, abundance and size-structure of cerithiid gastropods in sediments at One Tree Reef, southern Great Barrier Reef

Abstract

The patterns of abundance of molluscs in sediments at One Tree Reef, southern Great Barrier Reef were determined for three adjacent habitats in October 1985: a shallow subtidal sandflat, a deep channel, and the sloping, unstable edge separating the two. Deposit-feeding cerithiid gastropods, especially Rhinoclavis aspera (Linnaeus, 1758), R. fasciata (Bruguière, 1792) and R. vertagus (Linnaeus, 1758), were the most abundant molluscs present but these species were predominantly found in the coarse sandy sediments of the sandflat. The abundances and sizes of these three species were monitored seasonally from October 1985 to January 1988 on the sandflat. Densities fluctuated during the three years of the study and varied at a number of spatial scales. The density of R. aspera was generally greater at the beginning of the year, after recruitment, then gradually declined until the next period of recruitment. This pattern was not as evident for R. fasciata or R. vertagus. Changes in size-frequency distributions through time suggest that movement of animals, either by migration and/or passive transport in the shifting sediments may contribute to the observed patterns. Comparisons are made between temperate and tropical soft-sediments habitats, especially in relation to the distribution of deposit-feeding organisms.