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."