Grey unicornfish

Lore & Background

Naso caesius was first formally described in 1992 by American ichthyologist John Ernest Randall and marine biologist Lori Jane Bell Colin, with the type locality given as the northern side of the East Channel of Eniwetak Atoll in the Marshall Islands. It is classified within the nominate subgenus of the genus Naso, which is the only genus in the subfamily Nasinae within the family Acanthuridae. The specific name caesius is Latin for 'bluish grey', referring to the overall colour of this fish.

The grey unicornfish has a dorsal fin supported by 6 or 7 spines and between 27 and 30 soft rays, while the anal fin has 2 spines and between 28 and 31 soft rays. The pectoral fin contains 16 to 18 fin rays, with the upper two being simple and unbranched. The keels on the caudal peduncle are oval shaped with spines that project outward but are not acute. The overall colour is bluish-grey or greyish-brown, and the fish can create a series of irregular blotches or bars along the flanks, which may be lighter or darker than the background colour. The maximum published standard length is 45.6 cm.

This species is found mainly in the western Central Pacific, including Palau, Mariana Islands, northern Marshall Islands, Hawaiian Islands, Great Barrier Reef, Osprey Reef, Chesterfield Islands, New Caledonia, Fiji, Tuvalu, Society Islands, and Pitcairn Islands. In Australian waters, it occurs at Lord Howe Island, reefs in the Coral Sea as far south as the Kurnell Peninsula in New South Wales, off Western Australia, and at Ashmore Reef, Christmas Island, and the Cocos (Keeling) Islands. It is a benthopelagic species found over rock and coral substrates, occurring solitarily or in schools in clear waters of lagoon reefs, reef channels, or seaward reefs at depths between 15 and 50 m, where it feeds on zooplankton.

Reader's Guide

The grey unicornfish is a member of the family Acanthuridae, which includes surgeonfishes, unicornfishes, and tangs. Its description in 1992 by Randall and Colin added to the known diversity of the genus Naso, the only genus in the subfamily Nasinae. The species is distinguished by its bluish-grey colour and the oval, non-acute keels on its caudal peduncle. Its distribution across the Indo-Pacific, from the Marshall Islands to the Great Barrier Reef and as far as the Pitcairn Islands, highlights its adaptability to clear-water reef environments. The fish occupies benthopelagic habitats over rock and coral substrates at depths of 15 to 50 m, where it feeds on zooplankton, either alone or in schools. Its presence in both lagoon and seaward reefs underscores its ecological role in these ecosystems. The species' maximum standard length of 45.6 cm indicates a moderately sized unicornfish. The specific name caesius, meaning 'bluish grey', directly references its coloration, which can include irregular blotches or bars. This fish contributes to the understanding of reef fish diversity and distribution in the Indo-Pacific region.

Did You Know?

• The grey unicornfish was first formally described in 1992 by John Ernest Randall and Lori Jane Bell Colin.
• It feeds on zooplankton and occurs at depths between 15 and 50 m.