RECRUITMENT OF CERITHIID GASTROPODS (RHINOCLAVIS SPP) IN SEDIMENTS AT ONE TREE REEF, GREAT-BARRIER-REEF

The abundance of three species of deposit-feeding cerithiid gastropods, Rhinoclavis aspera (Linnaeus, 1758), R. fasciata (Bruguiere, 1792) and R. vertagus (Linnaeus, 1758), was monitored seasonally from May 1986 to January 1987 in sediments at One Tree Reef, southern Great Barrier Reef. All three species were most abundant in the coarse sandy sediments of a shallow sandflat, rather than in the finer sediments found in deeper channels within the lagoon. In contrast deposit-feeding temperate zone infauna are typically more abundant in finer sediments. To evaluate whether this pattern of greater abundance in sandy sediments was a result of greater recruitment to the sandflat compared with the channels, recruitment of each species was determined in each of two successive summers, January 1987 and January 1988. R. aspera recruited mostly to the sandflat compared with other habitats in both of 1987 and 1988, but for R. fasciata and R. vertagus this was only the case in 1988. These results are consistent with a model of larval depletion in the water column because larvae are always carried across the sandflat by water currents before reaching the lagoonal habitats so there are fewer larvae left in the water to settle into the finer sediments of the lagoon. The effect of removing all adult macrofaunal molluscs from the sediment on the recruitment of each species was also experimentally determined at 10 locations. The results varied among locations supporting alternative models of adult enhancement of recruitment, reduction of recruitment due to adults, or no effect of adults on recruitment. The results of this experiment, however, provide evidence against the previous larval depletion model because in some cases recruitment was increased in the lagoonal habitats where adults were removed indicating that there were still larvae available in the water over the fine sediments.