Whitemargin Unicornfish

The Whitemargin Unicornfish (Naso annulatus), commonly referred to as the Ringtailed Unicornfish or Short-horned Unicornfish, is a notable tropical species distributed across the Indo-Pacific region. Distinguished as one of the largest species within the Acanthuridae family, it can achieve a length of up to 100 cm.

Initially described in 1825 as Priodon annulatus by French naturalists Jean René Constant Quoy and Joseph Paul Gaimard, its type locality was identified as Timor Island. This species is classified within the nominate subgenus of the genus Naso, which falls under the subfamily Nasinae of the family Acanthuridae.

The Whitemargin Unicornfish exhibits a dorsal fin supported by five spines and 28 or 29 soft rays, while the anal fin comprises two spines and 27 or 28 soft rays. The body length ranges from 2.2 to 3 times its depth, with maximum depth observed in adult specimens. A prominent feature is the elongated, horizontal bony protuberance located anterior to the eyes, appearing as a bump in individuals under 20 cm in length. The angle between the snout and this protuberance is approximately 60°. The caudal peduncle is equipped with two bony plates on each side, evolving into blade-like keels in adults. The caudal fin transitions from a truncate form in juveniles to a scalloped shape in adults, with males developing extended filaments at the lobe tips. The body exhibits a bluish-grey or bluish-brown coloration, lighter on the lower body, without dark markings. Juveniles and subadults feature distinctive white markings, including a band on the caudal peduncle and an edge on the caudal fin. Upon maturation, a black submarginal band and a thin white margin persist in the caudal fin, complemented by tail filaments. With a maximum total length reaching 100 cm, it holds the distinction as the largest species within both the genus Naso and the Acanthuridae family.

The Whitemargin Unicornfish demonstrates an extensive distribution across the Indo-Pacific, ranging from the 🌊 Red Sea and eastern African coasts to 🇨🇨 Cocos Island (🇦🇺 Australia) in 🇨🇷 Costa Rica. Juveniles inhabit shallow, clear waters of lagoon reefs, beginning at depths as shallow as 1 m, while adults are typically found at depths exceeding 25 m along outer reef drop-offs.

This species tends to form small schools. Juveniles primarily consume filamentous green algae, whereas adults, exceeding 20 cm in length, shift their diet to soft zooplankton, including ctenophores and jellyfish. Reproductive behavior involves males and females forming pairs for spawning.