Unicornfish are marine fish making up the Naso genus of the Acanthuridae family. They are closely related to tangs and surgeonfish, both belonging to the same family as unicornfish. Sometimes unicornfish are referred to as unicorn tangs. There are at least 20 species of unicornfish, and they are named after the horn-like protrusion on their head between the eyes. The purpose or function of the horn is not understood, and not all species have this protrusion. Some species have a small bump on their forehead, while other species have no protrusion of any kind. For example, the bluespine unicornfish develops a horn while the orangespine unicornfish does not. For species with the horn, the appendage starts growing when young fish typically reach about 13 cm in length. Males also tend to have bigger horns. Depending on the species, unicornfish can grow to be as big as 50-61 cm in length. They have a laterally flattened body and smooth skin. They’re also less susceptible to skin disease than tangs are. Unicornfish have a lifespan of about 15 years. They are mainly active during the daytime.
Unicornfish strictly inhabit the tropical regions of the Indian and Pacific oceans. They have also been spotted around Hawaii, Japan and the Red Sea. Their preferred habitats are coral reefs, patchy reefs and rocky areas, but a species’ diet will determine where they’re more likely to be found. Bottom-dwelling algae grazers, like the bluespine and orangespine unicornfish, will more likely be seen on shallow reef surfaces. Plankton feeding species, like the paletila and sleek unicornfish, will be found further out in the pelagic zone at depths as far as 700 ft from the reef. Unicornfish are generally herbivorous.
Since their horn is not a consistent feature, they are more easily identified by the double-set of sharp scalpels at the caudal peduncle or tail base. These two fixed blades are modified scales on either side of the tail, contrasting the single retractable blade of the surgeonfish. They utilize these scalpels as a defense against predators and a weapon when fighting to ward off breeding competition, instead of their horns. Some species like the bluespine and orangespine unicornfish have brightly colored blades that contrast with their body. This serves as a warning to both predators and competition. Unicornfish are usually not aggressive towards each other otherwise. While sometimes solitary or in pairs, many can be seen roaming in groups together. They’re not aggressive towards humans either, but it’s suggested that divers do not chase or corner unicornfish because their scalpels can cause deep cuts. They are a valued food fish in Hawaii and their tough skin is used for making drumheads. Hawaiians prefer hook and line reef fishing methods for capturing unicornfish. The catch rates remain fairly low with this method, but the bycatch also remains minimal. Most species of unicornfish fish either seem of “least concern” or “not evaluated,” like the bignose unicornfish, on the International Union for Conservation of Nature (IUCN) red list.