Abstract:

The acanthurid fish species Ctenochaetus striatus and Acanthurus nigrofuscus are common detritivore browsers. Less is known about their erosion potential. We offered coral limestone for grazing in aquarium experiments and recorded the feeding behaviour by video. C. striatus produced 15 times more calcareous sediments compared to A. nigrofuscus despite of a 5 times higher biting rate of A. nigrofuscus. Whereas A. nigrofuscus performed careful picking bites only, C. striatus showed soft sweeping of the surface alternating with chafing the substrate with grasping bites. Hereby a special palate structure (bow of hard knobs in the upper jaw) is engaged which allows to rasp the reef substrate. The measured eroded masses per bite and the number of bites per individual were related to the population densities of the two species on three reef crests in the Northern Red Sea. Based on these extrapolations, A. nigrofuscus attains maximum erosion rates of 2.6 g m-2 yr-1 and may therefore not be regarded as a relevant bioeroder, but C. striatus achieves erosion rates of up to 70.0 g m-2 yr-1 which is in the same order as that of the co-occuring sea urchin Diadema setosum.