Intra- and inter-specific competition for food in infaunal coral reef gastropods

Abstract

Deposit-feeding gastropods have mostly been studied in the temperate northern hemisphere where these animals are typically found in fine muddy sediments, presumably as a response to the amount of food available. At One Tree Reef, in the southern Great Barrier Reef, deposit-feeding cerithiid gastropods, Rhinoclavis aspera (Linnaeus, 1758), R. fasciata (Bruguière, 1792) and R. vertagus (Linnaeus, 1758), are most abundant in an atypical environment, the coarse sandy sediments of a back-reef sandflat. Densities in the finer sediments of the lagoon are several orders of magnitude smaller than on the sandflat. In this study, these three species were enclosed at a range of naturally occurring densities in fine and coarse sediments to investigate their competitive interactions and the influence of sediment on their growth. The type of substratum in experimental enclosures influenced the growth of each species. Growth was greatest in the fine sediments from the lagoon, despite the fact that recruitment of each species had been shown to be greatest into the coarse sediments of the sandflat. Adult R. aspera significantly reduced the growth of juveniles of the three species, but adults of R. fasciata and R. vertagus had no significant effect on the growth of juveniles of R. aspera. These results are probably a function of the large disparity in the densities of the three species: R. aspera occur in densities an order of magnitude greater than the other two species and naturally occurring densities were used in the experiments. Competition for food or other expendable resources was distinguished from direct interference competition for space by including in our experiments treatments with dead shells occupying the same volume as live adults, but not utilizing expendable resources. Direct competition for space between adults and juveniles of R. aspera is unlikely to be important as a factor affecting the growth of juvenile R. aspera. Dead shells had no significant effect on the growth of juveniles in coarse or fine sediment. Direct competition for space may be more important for R. fasciata and R. vertagus because, in some cases, the presence of dead shells reduced the growth of juveniles. These experiments demonstrate how complex competitive interactions among Rhinoclavis spp. are modified by the type of sediment and vary with the particular combination of competitors. Research in rocky intertidal systems has similarly shown how the physical environment may modify the nature of biological interactions such as competition and predation. Our results are consistent with findings from temperate systems in that growth of Rhinoclavis spp. was limited by the availability of food or a related resource such as the availability of suitably sized particles of sediment for ingestion. This system differs, however, because deposit-feeding snails occur in their greatest densities in a habitat where they have the slowest rates of growth. Further work is needed to explain these patterns of distribution and abundance and why they differ from those typical of studies in temperate systems in the northern hemisphere.