Benthic cryptofauna and internal bioeroders on coral reefs

Abstract: The cryptofauna of coral reefs may act as a food source for fish, while being grazers or predators themselves, and hence constitute an important group in the coral reef food web. In addition, the actively eroding taxa of the cryptofauna contribute to the decomposition and recirculation of calcium carbonate and are consequently essential for the rebuilding new coral. However, when coral reefs are disturbed, changes in these functions may severely affect the reef ecosystem. In this thesis correlations and causal effects of environmental factors on benthic cryptofauna and bioerosion of corals were investigated at several coral reefs off the west coast of Zanzibar (Unguja), Tanzania. An aim in the first paper was to investigate effects of elevated nutrient levels on bioerosion and bioeroding fauna. Yet, the study found no difference in abundance of the total eroding fauna with distance to a major sewage outlet. However, the faunal composition differed along the gradient, with some deposit feeders being more numerous near the outlet and some suspension feeders being more numerous further away. In a comparison of three coral taxa, as well as coral rubble, the total boring fauna did not differ significantly in abundance or erosion.Internal bioerosion generally increases with time, and the second paper focused on the succession of bioeroders at two experimental sites. There was more total internal erosion at one of the sites, and some eroding taxa were common at one site while rarely found at the other. While some taxa followed an expected successional pattern, others did not. Within taxa, number of individuals was uncorrelated to the extent of erosion, suggesting that it is a result of increased growth rate and low recruitment of new individuals.The last two papers deal with the influence of fish grazing of algae on the bioeroding and non-eroding cryptofauna, respectively. The bioeroding community showed a complex response to varying grazing pressure, varied within and outside damselfish territories, with some taxa, usually suspension feeders, being more abundant in highly grazed areas outside damselfish territories while deposit feeders were more abundant within territories. Further, with a high grazing pressure, the non-boring cryptofauna decreased in abundance compared to areas protected from grazing. However, this was not true for all taxa. Comparing the abundance of taxa at the surface of the coral with taxa in the coral holes and cavities, none of these correlated in numbers.