Bluespine Unicornfish

The Bluespine Unicornfish is a large tang in the family Acanthuridae, notable for the bony, horn-like protuberance that projects from the forehead of adults, giving the genus Naso its 'unicornfish' common name. It ranges widely across the Indo-Pacific, from the Red Sea and East Africa to Hawaii and the eastern Pacific, and is among the largest and most culturally significant reef herbivores in many Pacific Island communities. Rather than the retractable spine seen in Acanthurus species, it carries a pair of fixed, keeled bony plates on the caudal peduncle. It is currently assessed as a species of conservation concern in parts of its range due to targeted harvest pressure.

Recognize the Bluespine Unicornfish by these features:

• Large, elongate oval body in olive-grey to blue-grey
• A forward-projecting bony horn on the forehead, present in adults and larger juveniles
• Two fixed, blue-white keeled bony plates on each side of the caudal peduncle, in place of a folding spine
• Lunate tail fin
• Thick, somewhat protruding lips

Adults can exceed 60-70 cm, making it one of the largest surgeonfish species. The forehead horn and paired fixed tail-base plates readily separate it from horn-less Acanthurus species and from the smaller-horned Orangespine Unicornfish.

Bluespine Unicornfish inhabit coral reefs, reef flats, and adjacent seagrass or algal-covered rocky areas throughout the tropical Indo-Pacific, from the Red Sea and East Africa to Hawaii and islands of the eastern Pacific. They are typically found from the shallows down to about 80 m, with juveniles often settling in shallow surge channels and reef flats before moving to slightly deeper reef habitat as adults. The species favors areas with abundant brown macroalgae, its primary food source, and is frequently encountered near reef edges exposed to moderate current and wave action across its broad geographic range.

This large tang feeds primarily on brown macroalgae, using its thick lips to browse fronds directly rather than scraping fine algal turf as many smaller surgeonfish do. Individuals may be solitary, paired, or found in loose feeding groups, and larger schools sometimes form for spawning. When threatened, it can slash sideways with its sharp fixed caudal plates, a defense also used in disputes with other unicornfish. Spawning follows a broadcast pattern, often timed to lunar cycles, with large aggregations gathering at reef passes to release eggs and sperm into passing currents, aiding wide larval dispersal before juveniles settle back onto reef habitat.