The feeding and defecation behaviour of the surgeonfish Ctenochaetus striatus was investigated at Ras Mohammed National Park (South Sinai, Red Sea). The fish feed on coral rock mainly by sweeping loose sediment with their flexible broom like teeth into their mouths. Feeding occurred exclusively on coral rock, but defecation took place only outside the grazing area above sand in small, precisely defined areas.
Excessive sedimentation is a major threat to coral reefs. It can damage or kill reef-building corals and can prevent the successful settlement of their planktonic larvae. The surgeonfish Ctenochaetus striatus feeds on rocky surfaces by sweeping loose material into its mouth with its flexible, broom-like teeth. In addition, it grasps and removes hard substrates with the aid of its special palate structure. It then transports sediment matter of the reef by defecating the ingested material outside the rocky zone of the reef. We analyzed 150 feces samples of six individuals, diferentiating between (1) ingested by sweeping and (2) ingested by scraping, and compared their content with inorganic land-derived and marine sediments trapped at the feeding area. Projections based on fish densities, defecation rates, and quantities as well as composition of sediments collected by traps on the same reef site suggest that C. striatus removes at least 18% of the inorganic sediment sinking onto the reef crest. The eroded share in the exported matter is about 13%. This finding points to a hitherto not verified role of C. striatus as a reef sweeper and reef scraper, whereby the first function is by far dominating.
A 15 N tracer study was performed during an experimental transplantation trial of natural Posidonia oceanica cuttings. The experiment was done in situ at 17 m depth in the Revellata Bay (Calvi, NW Corsica, France). Despite high survival rates of transplants (>90%) after one year, the weight and the N content of transplants are significantly lower than those of reference plants. In absence of roots, the transplants are not able to meet their N requirement because, leaf uptake is insufficient to replenish the N lost during the natural leaf decay. This could constitute a major cause of long-term failure for transplantation experiments or natural re-colonisation processes. The increase of the 15 N content in the roots shows that the plant re-allocates the nitrogen of one organ (i.e. leaves, rhizomes) to ensure the growth of another (i.e. roots).
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