Unicornfish

The Unicornfish (Naso unicornis) is the type species of its genus and one of the largest herbivorous surgeonfish on Indo-Pacific reefs, named for the distinctive bony horn that projects forward from its forehead as it matures. Also called the bluespine unicornfish for the pale blue spines flanking its tail base, it plays an important ecological role grazing tough, leathery brown algae that many other herbivores avoid, helping keep reef surfaces clear for coral growth. Juveniles lack the horn entirely and develop it gradually with age. The species is found from the Red Sea across the Indo-Pacific to Hawaii and is an important subsistence and commercial reef fish across much of its range.

• Horn: Bony, forward-projecting horn on the forehead, absent in juveniles and small in early adults, growing longer with age.
• Body: Deep, oval, olive-gray to brownish body, often darkening near the tail.
• Tail spines: Pair of pale blue scalpel-like spines at the caudal peduncle, giving rise to the alternate name 'bluespine unicornfish'.
• Tail shape: Lunate (crescent) tail, sometimes with trailing filaments in large adults.
• Look-alikes: Distinguished from other Naso species by the specific horn shape and lack of the elongated snout filament seen in Naso brevirostris.

Unicornfish inhabit shallow reef flats, lagoons, and seaward reef margins throughout the Indo-Pacific, ranging from the Red Sea and East Africa to Hawaii and French Polynesia. They are most commonly found in surge-exposed shallow water from the surface down to about 8 meters, though they can occur somewhat deeper on outer reef slopes. Juveniles often shelter in shallow tide pools and reef flat channels with strong wave action, while adults range more widely, sometimes forming loose foraging groups over algae-covered rock and reef pavement. They favor areas with abundant brown macroalgae growth, which forms the bulk of their diet.

Unicornfish are important grazers of coarse brown algae, using their strong jaws to strip leathery seaweed from rock and reef surfaces that many other herbivorous fish cannot process. They often forage alone or in small loose groups, moving across reef flats and shallow surge zones during the day. Like other surgeonfishes, they possess sharp caudal spines used defensively against predators and rival fish. Some populations undertake local migrations to specific spawning aggregation sites, where large numbers gather seasonally around the new or full moon to release eggs and sperm into open water, a behavior that has made spawning aggregations a focus of fisheries management in parts of the Pacific.