Fishes · Bony fishes · Unicornfishes

Bluespine Unicornfish

Naso unicornis (Forsskål, 1775)

syn. Acanthurus unicornis, Chaetodon unicornis, Naso unicornus

50 - 70 cm1-180 mLeast Concern

1205

The Bluespine Unicornfish, known scientifically as Naso unicornis, is a tropical marine fish associated with coral reefs. It thrives at depths ranging from 1 to 180 meters (3 to 590 feet). Fully grown, this fish can reach a maximum length of 70 centimeters (about 28 inches), although it is commonly found at around 50 centimeters (about 20 inches). The oldest known individuals can live up to 55 years.

In the wild, adult Bluespine Unicornfish typically inhabit channels, moats, lagoons, and seaward reefs where the water surges strongly. They are generally found swimming in small groups but can also be solitary. Juveniles tend to stay in shallow, protected areas like bays and harbors. These fish are active during the day and primarily feed on coarse leafy brown algae such as Sargassum. They reproduce through pair-spawning.

The physical characteristics of the Bluespine Unicornfish are distinctive. Adults have a tapering, bony horn on their forehead, which starts as a bump when they are about 12 cm (5 inches) long. This horn projects at the level of the eyes but does not extend past the mouth. The fish has six dorsal spines, between 27 and 30 dorsal soft rays, two anal spines, and 27 to 30 anal soft rays. The body depth changes from about twice the standard length in subadults to 2.4-2.6 times in adults. The tail fin, or caudal fin, is emarginate in youth and becomes truncate with filamentous lobes as they mature. Males typically develop longer horns and larger peduncular features than females of the same size.

Color-wise, the body of the Bluespine Unicornfish is yellowish to olivaceous gray on top and paler underneath, with distinctive blue peduncular plates and spine keels. Their lips are whitish or blue, and some areas around the pectoral fin are also blue. The dorsal and anal fins are yellowish with narrow blue edges, while the caudal fin is orange at the base, transitioning to gray, with a broad, pale greenish back edge trimmed with blue filaments. A noticeable transient color phase includes a blotchy pale greenish zone below the dorsal fin.

Geographically, the Bluespine Unicornfish is found throughout the Indo-Pacific, ranging from the 🌊 Red Sea and East Africa to the Hawaiian, Marquesas, and Tuamoto Islands (🇵🇫 French Polynesia). Its range extends north to southern 🇯🇵 Japan and south to the Lord Howe and Rapa Islands, with the potential presence in 🇸🇴 Somalia requiring confirmation. The genus name "Naso" is derived from the Latin word for nose, referencing the prominent horn on its forehead.

Why it's threatened

Biological resource use

Intentional use: (subsistence/small scale) [harvest]

Naso unicornis is a targeted food fish. There have been significant reductions in biomass in parts of its range.

Surgeonfishes show varying degrees of habitat preference and utilization of coral reef habitats, with some species spending the majority of their life stages on coral reef while others primarily utilize seagrass beds, mangroves, algal beds, and /or rocky reefs. The majority of surgeonfishes are exclusively found on coral reef habitat, and of these, approximately 80% are experiencing a greater than 30% loss of coral reef area and degradation of coral reef habitat quality across their distributions. However, more research is needed to understand the long-term effects of coral reef habitat loss and degradation on these species' populations. Widespread coral reef loss and declining habitat conditions are particularly worrying for species that recruit into areas with live coral cover, especially as studies have shown that protection of pristine habitats facilitate the persistence of adult populations in species that have spatially separated adult and juvenile habitats (Comeros-Raynal et al. 2012).

Threat classification from the IUCN Red List.