The distribution and abundance of boring species of polychaetes and sipunculans in coral substrates in French Polynesia

The distribution and abundance of the dominant initial macroborers of dead coral substrate, sipunculans and polychaetes were investigated over time at seven sites within French Polynesia. Sites were located in the lagoon of high islands and atolls, and varied from highly eutrophic to oligotrophic. Significant differences occurred between sites and patterns of recruitment varied over time and between sites. With increasing exposure, the densities of polychaetes increased but not the number of species present, whereas both the densities and number of species of sipunculans increased. The atoll sites tended to be dominated by suspension feeding polychaetes and the high island sites by deposit feeding polychaetes. Sipunculans tended to dominate the high island sites in comparison to the atoll sites and they all fed by scraping algae and detritus from the substrate. We suggest that this distribution of feeding types is related to water quality and to land run off. In the atolls, the lagoonal waters are oligotrophic and little land run off occurs, whereas at the high island sites, high rates of land run off occur during the wet season with high levels of suspended material in the water column.

These variations in densities of boring species, affect rates of bioerosion and have the potential to alter the equilibrium between reef growth and reef destruction. We suggest that it is critical for reef managers to try to maintain water quality and limit land-based terrestrial run off and associated nutrients into coastal waters. This is especially important if the reefs have been affected by bleaching events or Crown of Thorns plagues, resulting in extensive death of coral colonies and with it, the potential for a massive increase in the rate of bioerosion. The long-term maintenance of the reef structure is critical if coral recruitment and recovery of the reef are to occur.