Orangeshoulder Tang

The Orangeshoulder Tang (Acanthurus olivaceus) is a gray-bodied surgeonfish easily identified by the bright orange, teardrop-shaped patch sitting just behind its gill cover, sometimes called an 'epaulette' marking. It is found across the Indo-Pacific on reef edges and adjoining sandy or rubble areas, where it grazes algae and organic detritus from open substrate. Juveniles are entirely bright yellow and look markedly different from adults, gradually transitioning to the gray adult coloration with the orange shoulder patch as they mature. The species is a common sight along reef-sand interfaces and is a popular, relatively hardy aquarium fish valued for its algae-grazing habits and distinctive marking.

• Shoulder patch: Bright orange, teardrop-shaped mark just behind the gill cover, unique to this species among similarly shaped tangs.
• Body color: Uniform gray to olive-gray in adults, contrasting with the orange patch.
• Tail: Blue-outlined caudal fin margin.
• Juveniles: Entirely bright yellow, lacking the orange shoulder patch and gray body of adults.
• Look-alikes: Adults can resemble other plain gray Acanthurus species, but the orange shoulder patch is diagnostic; juveniles may be confused with other yellow juvenile tangs before the patch develops.

Orangeshoulder Tangs range across the Indo-Pacific from the eastern Indian Ocean through Indonesia and the Philippines to Micronesia and the central Pacific. They are commonly found along reef margins bordering sandy or rubble flats, typically at depths from about 3 to 46 meters. Adults often patrol the interface between hard reef structure and open sandy substrate, while juveniles tend to stay closer to sheltered patch reefs and coral rubble for protection. The species favors clear tropical water with a mix of algae-covered rock and open sand for foraging.

Orangeshoulder Tangs feed primarily by grazing filamentous algae and fine organic material from sand and rubble surfaces, often stirring up loose sediment as they forage along reef-sand borders. They may be seen singly, in pairs, or in loose foraging groups, particularly over productive sandy grazing grounds. As with other surgeonfish, individuals defend small feeding areas using their sharp caudal spine when challenged by rivals. The dramatic color change from bright yellow juvenile to gray-bodied, orange-shouldered adult occurs gradually over the fish's first year. Spawning is pelagic, with adults releasing eggs and sperm into open water, typically at dusk, producing planktonic larvae that eventually settle onto reef habitat.