Abstract

An immaculate square-nosed acanthurid fish of the genus Naso, previously misidentified as N. tuberosus, is described as a new species, N. mcdadei. The species is widespread, but appears uncommon, being known from only 10 specimens and five underwater photographs. It is described from specimens from tropical eastern and western Australia, southern Indonesia, Mauritius, and Natal, South Africa. Its occurrence is confirmed from off western Sumatra, the Maldives, Chagos Archipelago and southern Taiwan by photographs. It differs from N. tuberosus (Lacèpede, 1801) and N. tonganus (Valenciennes, 1835) most notably through smaller, more numerous teeth and a generally uniform greyish colouration. The head, body and fins all lack numerous small dark spots that are variously present in the latter two species. In adults, it also differs through a squarish rather than rounded tuberosity on the snout, a shorter straighter nasal groove, lack of a discernible hump in the dorsal profile, smaller-scale tubercles, non-prominent jaws, and first rather than last dorsal spine longest.

The species previously recognised as N. tuberosus is split into two closely related taxa, N. tonganus (Valenciennes) from the western Pacific and Indian Oceans and N. tuberosus Lacèpede from the central and western Indian Ocean. These species differ mainly in the size and shape of the nasal tuberosity, dorsal profile, body depth, and colouration.