Parrotfish, Great Barrier Reef
Photograph by David Doubilet, National Geographic
The clownish grin of a bridled parrotfish reveals its power tools: grinding teeth used to scrape algae from rock. Though sometimes destructive to individual corals, the fish's efforts are mostly beneficial. Without them, algal growth could smother the reef. Scarus frenatus
See more pictures from the May 2011 feature story "A Fragile Empire."