A coral-feeding butterflyfish with fine diagonal lines and a red anal fin margin.
To about 14 cm, the body dusky purple-grey and crossed by fine diagonal orange-brown lines that run up and back, with a black bar through the eye, a yellow-orange belly and a red to orange margin along the anal fin, which is where the name comes from. The Indian Ocean carries a close relative, Chaetodon trifasciatus, in the same role, and the two meet around western Indonesia.
It is an obligate coral feeder, taking the polyps of Acropora and Pocillopora and little else, so it is only where there is living hard coral and it thins out first when a reef is damaged. Pairs are stable and stay together over years, working a patch of coral in company and rarely straying far from it. On a muck destination it belongs to the coral site rather than the sand, and is met on the reef edge at the end of a slope. Recorded from about 2 to 25 m.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.