A Powderblue Surgeonfish in a fish tank at Chatuchak Market. This fish derives its name from two hard spines at the base of its tail, one on each side of the body, that lie flat in a groove and which are as sharp as a surgeon's scalpel. When the fish is provoked or feels threatened, these spines can be used as a defensive weapon, as when the fish flips its tail, the spines pop out like small scalpel-like knives.