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 off the reef by defecating the ingested material outside the rocky zone of the reef. We analyzed 150 feces samples of six individuals, differentiating 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.
Coral reefsRed SeaSedimentationFeeding and defecation behaviorReef sweeper
SOM Supporting online material. Short summary: By feeding on loose particles accumulated on the top of coral reefs and defecating in a restricted place adjacent to its feeding area, the surgeon fish Ctenochaetus striatus transports loose particles from the reef, and hence functions as a “reef sweeper”. Movie comment: The tropical reef fish Ctenochaetus striatus (surgeonfish) visits specific places for defecation on sandy bottom beside the rocky reef. Ras Mohammed National Park, Sinai, Egypt, Red Sea. The fish stops its normal grazing activities on the reef rock and immediately starts to swim taking the shortest way over the edge of the reef where it stops about 20 cm above the adjacent sand-covered reef floor. The fish then assumes an oblique position with its head upwards and spreads its pectoral fins and deposits a fecal pellet. After this, it returns to the rocky zone to continue grazing. (MOV 6302 kb)