Whitemargin Unicornfish

The Whitemargin Unicornfish, scientifically known as Naso annulatus, is also called the Ringtailed or Short-horned Unicornfish. This tropical species is one of the largest members of the Acanthuridae family, reaching up to 100 cm (about 3 feet) in length. It inhabits the vast Indo-Pacific region, from the 🌊 Red Sea to the coasts of eastern Africa and as far as 🇨🇨 Cocos Island (🇦🇺 Australia) near 🇨🇷 Costa Rica.

First described in 1825 by French naturalists Quoy and Gaimard, the type locality of this species is Timor Island. The Whitemargin Unicornfish is part of the Naso genus within the Nasinae subfamily of the Acanthuridae family.

The fish is characterized by a dorsal fin supported by five spines and between 28 to 29 soft rays, while its anal fin has two spines and 27 to 28 soft rays. The body is 2.2 to 3 times longer than it is deep. A notable feature is the bony protuberance on the head, appearing as a bump in individuals less than 20 cm (around 8 inches) long. As the fish matures, the caudal fin changes from a straight to a scalloped shape, with males sporting long filaments at the tips. The body color is typically bluish-grey or bluish-brown, lighter towards the bottom, without dark markings. Younger fish have striking white markings, including a band on the tail and a white edge on the fin. As they mature, they retain a black band and thin white margin on their tails along with filament extensions.

Whitemargin Unicornfish are commonly found in coral reefs. Juveniles prefer shallow waters as shallow as 1 m (3 feet), while adults inhabit depths of more than 25 m (82 feet) along reef drop-offs. These fish form small schools and exhibit a dietary shift with maturation. Juveniles consume green algae, but adults over 20 cm in length favor soft zooplankton like ctenophores and jellyfish. For reproduction, males and females pair up for spawning.