The Threadfin Butterflyfish, Chaetodon auriga, is a species of butterflyfish (family Chaetodontidae). It is found in the Indo-Pacific region from the Red Sea and eastern Africa (south to Mossel Bay, South Africa) to the Hawaiian, Marquesan, and Ducie islands, north to southern Japan, south to Lord Howe Island and Rapa Iti, at depths of between 1 and 35 m.
Length is up to 23 cm (9 in). Chaetodon auriga has a neck patch of ascending and a belly patch of descending oblique dark lines. Adults of this species have a prominent black vertical band running across the eye, and a filament trailing posteriorly from the upper soft portion of the dorsal fin. Juveniles have a black bar over the head that hides the eye and an eye-sized black spot on the soft dorsal fin. This spot is usually absent in Red Sea adults but retained elsewhere. As the name Chaetodon auriga originally refers to the Red Sea population, the spotted population is sometimes separated as subspecies Chaetodon auriga setifer.
It belongs to the large subgenus Rabdophorus which might warrant recognition as a distinct genus. In this group, it almost certainly is a rather close relative of the Vagabond Butterflyfish (C. vagabundus) and the Indian Vagabond Butterflyfish (C. decussatus). The Threadfin Butterflyfish might be closer to the latter; as C. vagabundus has yielded abnormal DNA sequence data this is hard to say however. The C. auriga species group shares the characteristic pattern of two areas of ascending and descending oblique lines; species differ conspicuously in hindpart coloration.
Threadfin butterflyfishes may be seen in a variety of habitats ranging from rich coral reefs to algae- and rubble-covered areas, either singly, in pairs, or in aggregations that roam over large distances in search of food. They feed mainly by tearing pieces from polychaetes, sea anemones, coral polyps, and algae. Reproduction is oviparous, with pairs forming during breeding.