Spotted unicornfish (Derek Keats / CC BY)

Unicornfish (Naso sp.) are relatively large members of the surgeonfish family. The unicornfish is named for a protruding spike or bulbous on its forehead that develops with age. Like other surgeonfish, unicornfish usually have razor-sharp spines near their tail fins, which they also use in self-defense. The whitemargin unicornfish can grow up to 1 meter (39 inches) in length.

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This supplemental online resource accompanies the book Scuba Diving Hand Signals by Lars Behnke, available on Amazon.