### Identification Look for a high-backed, pancake-thin profile that cuts a striking silhouette against the reef. Adults are a breathtaking deep cerulean to royal purple, often marked with fine, horizontal grey lines. The "smoking gun" for ID is the caudal spine—a bright yellow, scalpel-like blade at the base of the tail used for defense. Don't let the youngsters fool you; juveniles are a brilliant, solid canary-yellow with blue-fringed fins, transitioning through a "pastel" phase before reaching their iconic blue. Unlike the Indo-Pacific Regal Tang (*Paracanthurus hepatus*), *A. coeruleus* lacks black palette markings.
### Habitat & Range This is a quintessential Western Atlantic species. You’ll find them from the chilly waters of New York down to the Brazilian coast, though they are most abundant in the Caribbean and Gulf of Mexico. They frequent sun-drenched coral reefs and rocky inshore scrub at depths of 2 to 40 meters, where sunlight fuels the algae they depend on.
### Behaviour By day, these fish are restless movers. While they can be solitary, they are famous for forming "blue waves"—massive foraging schools that roam the reef. This schooling is a tactical maneuver; by swarming in high numbers, they overwhelm the defenses of territorial damselfish who try to guard their private algae patches. At night, they tuck themselves into reef crevices to sleep.
### Diet The Blue Tang is a vital "lawnmower" of the reef. They are strictly herbivorous, using their specialized teeth to scrape filamentous algae and detritus off coral carbonate and rocks. By keeping fast-growing algae in check, they prevent it from choking out the living coral polyps.
### Fascinating Fact The Blue Tang operates a high-end "spa" for Green Sea Turtles! They are frequently observed following turtles to pick off parasites and molting skin. They will even congregate at specific "cleaning stations" where turtles wait patiently while the fish graze across their shells.