Pacific Blue Tang, Paracanthus hepatus.

This fish from Indo-Pacific coral reefs should not be confused with the Atlantic blue tang so commonly seen by snorkelers in the Florida Keys or Caribbean, though both fish are members of the surgeonfish family.

About Surgeonfish:

Surgeonfish get their name from the sharp, erectable, scalpel-like spines on each side of the tail base.

(click for closeup of tail spines)

Surgeonfish slash these spines at rivals and other fish thtat threaten them. At feeding time, it is not uncommon to see our blue tang and sailfin tang (also a surgeonfish) slashing their tail spines at each other. Though some surgeonfish have very large, sharp spines that can inflict serious injuries to other fish, the most we ever see in our aquarium are minor scrapes inflicted by one tang on the other, and these are infrequent.