| Ecological Descriptors
Morphological and genetic data indicate that Acanthurus bahianus is now split into two species: A. tractus, which is distributed in the northwestern Atlantic and A. bahianus distributed from eastern Brazil to Ascension and St. Helena Islands
Body laterally compressed, oval in shape, with relatively long dorsal and anal fins and a crescent tail. Uniform in color without body bars. Can change from bluish gray to dark brown, and can pale or darken dramatically. Markings radiate from the eye. Dorsal, anal and tail fins with a blue or white border. Base of tail with a sharp spine (like a surgeon's scalpel), may have a pale band behind the spine. Pectoral fins clearish, sometimes with yellow tints. Sometimes pale band on caudal peduncle.
Inhabits shallow bottoms with coral or rocky formations, down to 40 m. Usually occurs in groups of five or more individuals. Mainly a diurnal species. Feeds mainly on algae, but occasionally on seagrass, it also feeds on the film of algae on the surface of sand undisturbed by surge. The sharp spines on both sides of the base tail are used as defensive weapons by slashing their tails from side to side. When not in use, they are folded backwards against the body. Schooling occurs primarily in adults; small juveniles never participate in large, dense schools. The proportion of adults that were schooling increased from the back reef to the reef crest and spurs. The pelagic juveniles of this species settle in sheltered shallow habitats. The pelagic larval stage ranges from 42 to 68 days, it then settles on reefs when 27 mm
A. bahianus displays the highly characterized pattern of asymptotic growth. Terminal size is reached at around age 4, and most of the growth occurred within the first 10% of the lifespan, and approximately 85% of full size is attained within the first year. It has a 1:1 sex ratio. Maturity seems likely to occur within 1 year in most cases at about 11 cm and most fish are probably mature at 15 to 16 cm. It has been observed to form a resident spawning aggregation of up to 20,000 individuals, spawning as subgroups. It also pair spawns, the males holding small territories adjacent to the aggregation site.