Meet our June Fish of the Month, the Blue Tang, Acanthurus coeruleus — a Caribbean reef icon with one of the most dramatic color changes in the tropical Western Atlantic. The juvenile pictured above is a vivid lemon yellow; the adult it will grow into is deep blue. Same fish, two completely different looks.

Survey Regions: Blue Tangs are found throughout the tropical Western Atlantic — from Bermuda and Florida through the Bahamas and Caribbean down to Brazil — including the Gulf of Mexico. REEF volunteers document them throughout the Tropical Western Atlantic (TWA) survey region, where they’re one of the most commonly sighted species on shallow reefs.

Size: Medium — up to about 39 cm / 15 in total length, though most individuals divers see are in the 20–30 cm / 8–12 in range. Newly settled juveniles are tiny, often just a few centimeters long.

Identifying Features: Adult Blue Tangs are unmistakable — an oval, laterally compressed body that’s deep blue to blue-gray, often etched with fine darker pinstripes running horizontally along the flanks. Look closely at the base of the tail and you’ll spot a small, sharp, often pale-yellow spine on each side: the scalpel-like caudal spine that gives the surgeonfish family (Acanthuridae) its name. Juveniles, like the one pictured, are a striking bright yellow all over, with a thin blue ring around the eye. As they grow, they pass through an intermediate phase — yellow body with blue fins — before settling into full adult blue. The transformation is so complete that early naturalists once thought juveniles and adults were separate species.

Fun Facts: First described in 1787 by German naturalist Marcus Elieser Bloch. The species name coeruleus is Latin for “sky blue” or “dark blue” — fitting for the adult, if not the juvenile. Blue Tangs are dedicated herbivores, grazing algae off the reef and playing a crucial role in keeping corals from being overgrown. They often forage in mixed-species groups with other surgeonfish and parrotfish; the sheer number of bodies overwhelms territorial damselfish that would otherwise defend their algae gardens against a single grazer. So next time you spot a flash of yellow zipping through a shallow Caribbean reef, take a closer look — you may be watching a future blue giant in its bright early days.

Photo by Tom Sparke, taken on the recent REEF Field Survey Trip to St. Eustatius.