Silverblotched 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, the only genus in the subfamily Nasinae within the family Acanthuridae. Its specific name caesius is Latin for 'bluish grey', referring to its overall colour.

The species has 6 or 7 spines and between 27 and 30 soft rays supporting the dorsal fin, while the anal fin is supported by 2 spines and between 28 and 31 soft rays. The pectoral fin contains between 16 and 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 benthopelagic species is found over rock and coral substrates, occurring solitarily or in schools in clear waters in lagoon reefs, reef channels, or seaward reefs at depths between 15 and 50 m, where it feeds on zooplankton. Its distribution includes the western Central Pacific, with records from 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 is known from Lord Howe Island, reefs in the Coral Sea as far south as the Kurnell Peninsula in New South Wales, as well as reefs off Western Australia, Ashmore Reef, Christmas Island, and the Cocos (Keeling) Islands.

Reader's Guide

The silverblotched unicornfish is notable as a relatively recently described species, first formally named in 1992 by Randall and Colin. Its description added to the known diversity of the genus Naso within the surgeonfish family. The species is distinguished by its bluish-grey colouration and the irregular blotches or bars it can display along its flanks, which may serve as camouflage or communication. Its distribution across the Indo-Pacific, from the Marshall Islands to the Great Barrier Reef and as far south as New South Wales, indicates a wide but patchy range. The fish occupies a specific ecological niche as a benthopelagic zooplankton feeder in clear lagoon and seaward reefs at moderate depths. Its presence in both the Pacific and Indian Oceans, including remote islands like Christmas Island and the Cocos (Keeling) Islands, highlights its adaptability to various reef environments. The species' maximum size of 45.6 cm places it among the larger unicornfishes, and its oval caudal peduncle keels with projecting but non-acute spines are characteristic of the genus. The etymology of its name, derived from Latin for 'bluish grey', directly reflects its most prominent visual feature.

Did You Know?

• The specific name caesius is Latin for 'bluish grey', describing the fish's overall colour.
• It was first formally described in 1992 by John Ernest Randall and Lori Jane Bell Colin.
• It feeds on zooplankton and is found at depths between 15 and 50 m.