Bioerosion of the coralline alga Hydrolithon onkodes by microborers in the coral reefs of Moorea, French Polynesia

Calcareous skeletons of the coralline alga Hydrolithon onkodes are colonised by a variety of microboring organisms including euendolithic algae. The species composition of microboring organisms as well as the boring patterns and the boring activity differ between live and dead coralline crusts. The microborers inhabiting the live crusts form an obvious, macroscopically visible green layer in the basal part of the crust, which is in contact with the substratum underneath. Some species, primarily Plectonema terebrans (83 %) and Ostreobium quekettii (7 %) grow from the inside towards the crust's surface and produce a dense network of branched filaments. Following the death of the coralline alga, the skeletons are colonised at the surface and bored inward by the cyanobacteria Hyella caespitosa, Mastigocoleus testartum, P. terebrans and various chlorophyta. The bioerosive activity was determined using Image Analysis. Carbonate removal was estimated at 0.12 g CaCO(3)(.)cm(-3) in the live crusts compared to 0.49 in dead crusts. Changes in endolith distribution and floristic composition between live and dead crusts, as well as subsequent changes in filament diameters of the microboring alga] communities and penetration depth of endoliths, could explain the significantly higher rate of bioerosion in dead crust versus live crust. Endolith activity was found to vary between live and dead crusts; while microborers are direct agents of bioerosion in both live and dead crusts by removing carbonate from the skeletons (8 to 32 % of the substratum, in volume), they indirectly increased bioerosion rate in dead crusts since they are themselves exposed to grazing by fish, echinoderms and molluscs. (C) 2001 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.