The threadfin butterflyfish, Chaetodon auriga, is a butterflyfish of the family Chaetodontidae found in the Indo-Pacific oceans from the Red Sea and East Africa (extending to Mossel Bay, South Africa) to the Hawaiian, Marquesan, and Ducie islands, north to southern Japan, south to Lord Howe and Rapa islands, at depths of between 1 and 35 m.
The threadfin butterflyfish has a pattern of “chevron” markings on the sides and a prominent black spot at the posterior edge of the soft portion of the dorsal fin. 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. Red Sea specimens usually lack the black spot at the end of the dorsal fin. Length is up to 23 cm.
They may be seen in a variety of habitats ranging from rich coral reefs to weedy 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.