The Whitemargin Unicornfish is truly a fascinating Reef Fish which is a delight to sight while scuba diving in the waters of the Indo-Pacific. These fish are known as Unicornfish because of a spike/horn that protrudes from its forehead which can grow quite prominently which makes this fish easy to identify and a curious inhabitant of the reef. The “Whitemargin” part of its name comes from the tail of the fish which is black surrounded by a white margin.
Whitemargin Unicornfish or Naso annulatus belong to the Tang or Surgeonfish family known as Annulatus. The distinctive characteristic of the Annulatus or surgeonfish family is the spines, one or more on either side of the tail, which are dangerously sharp. The Whitemargin comes from the “Naso” genus of the Annulatus family which comprise of 17 species of tropical marine surgeonfish’s all having some form of horn or protrusion from their foreheads. The word Naso in Latin means nose.
The Whitemargin unicornfish is found throughout the Indo-Pacific, commonly found in coral reefs, and rocky tropical areas of the Indian and Pacific Oceans and the Red Sea. Some are also said to be found near Hawaii and Japan. This species lives in the reefs at the depth range of 1 – 60 m i.e. up to 197 feet.
The Whitemargin can reach a length of 100 cm (3.3 ft) making it one of the largest members of the Surgeonfish family and a delight to divers, especially underwater photographers who spot an adult specimen on their dive.
These fish are mainly active in daytime and roam in groups feeding on algae. Being pure herbivores, their favorite food comprises of Leafy brown algae such as sargassum.
The horn-like appendage between their eyes begins growing when a young fish reaches about 13 cm (5 in) in length, and tends to be a little bigger on males. The fish never use this horn as a weapon or even for self-defense but instead use their sharp scalpel-like spines by their tails as a weapon.