Of all the Hawaiian reef fish, the surgeonfish or tangs are the most plentiful and prominent. At the base of their tail fin, they have two knifelike spines or scalpels on each side. These scalpels lie flat until they are ready to fight and they can quickly flip them out, which can cause serious injury to predators or rivals. The common name "surgeonfish" was from the deep and painful cuts they can inflict on humans, usually removing them from nets or spears. The scalpels are usually brightly colored, and peaceful divers have little to fear.
Surgeonfishes are usually oval shaped with narrow and compressed bodies, and their scales are very small. They launch themselves with winglike beats of their pectoral fins. Most of the surgeonfish feed on algae, and their mouths scrape the surface of rocks and dead coral or they nibble on leafy seaweeds. Since algae grows best in bright light, most surgeonfishes enjoy the shallow waters. There are 23 species of surgeonfish in Hawaiian waters.
The Achilles Tang is identified by a bright red teardrop-shaped patch over the scalpel, and its body is black. They commonly found along rocky shores with moderate wave action. They spend most of their time driving away intruders by turning sharply and exposing a scalpel. These fish take on a reddish glow if they are disturbed. They are named after the Greek warrior Achilles, who represents youthful grace, beauty and valor. They grow up to 10 inches.