The spine, called a caudal peduncle spine, is a modified scale anchored in a shallow groove either side of the tail. Muscular flexion of the peduncle erects it, and the fish delivers the blow with a lateral sweep of the tail rather than a bite. The edge is sharp enough to cut human skin, and handlers of aquarium stock are routinely warned about it.
Reference accounts of Paracanthurus hepatus describe the spine as venomous rather than purely mechanical. Animal Diversity Web and several public aquaria report a toxin associated with the spine, and the species is listed among venomous reef fish. The wound is still largely a laceration, and recorded human injuries are painful but minor, with infection the usual complication.
The gland anatomy is far better described for some Acanthurus and Naso species than for this one, and the potency of the toxin has not been well quantified. The bright yellow surrounding the spine in several surgeonfish is generally read as a warning signal. Anyone handling one should treat the tail as both a blade and a possible sting.