The blue tang is a surgeonfish, a group of fishes named for the scalpel-like spine on the side of the body, just ahead of the tail. Blue tangs, like all surgeonfishes, use these spines to defend against predators. They erect the sharp spines in an effort to make themselves harder to swallow or to inflict injury on their predators.
True blue tangs are restricted to coral reefs in the Caribbean Sea and surrounding waters and are often confused with two other surgeonfishes that inhabit the same waters, the doctorfish and ocean surgeonfish, as all three species look similar. Furthermore, another surgeonfish that lives in the tropical Pacific Ocean (and was made famous as Dory in the Finding Nemo movies) is sometimes incorrectly called a blue tang. That species (Paracanthurus hepatus) is more properly known as the regal tang or powder blue tang, and it does not closely resemble the blue tang, visually.
Blue tangs are herbivores, and they actively browse the surface of coral reefs, searching for their favorite algae. Though larger individuals sometimes browse by themselves or in pairs or threesomes, small to medium adults often form large groups and swim long distances, browsing along the reef surface throughout the day. With overfishing of their main predators (e.g., large groupers and snappers) and a reduction of some of their main competitors for algae (e.g., the Longspine Urchin), numbers of adult blue tangs can be quite high on many reefs. Juveniles live among dead coral rubble or in mangrove forests in more protected waters and move to the open reef surface as they mature. Adult blue tangs are solid blue (or almost dark purple), while juveniles are solid yellow.
Blue tangs reproduce through a behavior known as broadcast spawning, where several females release eggs and several males release sperm into the water column above the reef, all at the same time. This method increases the likelihood that eggs will become successfully fertilized and that fertilized eggs will not be eaten by egg predators on the reef surface.
Blue tangs are consumed by humans only rarely, when caught in artisanal fish traps, but bright yellow juveniles are targeted quite significantly in some locations for the private aquarium trade. Even so, scientists have assessed the blue tang’s population status and have found it to be a species of least concern (i.e., its numbers are currently fine).