Eastern Triangular Butterflyfish

The eastern triangular butterflyfish (Chaetodon baronessa), also called the eastern triangle butterflyfish or baroness butterflyfish, is a butterflyfish of the family Chaetodontidae. The World Register of Marine Species (WoRMS) lists Chaetodon baronessa Cuvier, 1829 as the accepted name, and FishBase gives eastern triangular butterflyfish as its English name. Wikipedia places it in the subgenus Gonochaetodon and warns against confusing it with the triangle butterflyfish (C. triangulum) or the triangulate butterflyfish (C. trifascialis).

FishBase gives a maximum length of 16 cm (total length) and a short, deep, compressed body. Wikipedia describes alternating cream and grey-brown to purple chevron-shaped bars on the body, and three dark bars on the head, one of them running across the eye.

FishBase lists the species as Least Concern on the IUCN Red List (version 2025-2, global; assessed 7 October 2009).

FishBase gives its range as the Western Pacific, from the Cocos-Keeling Islands in the eastern Indian Ocean and Indonesia to Fiji and Tonga, north to southern Japan and south to New Caledonia and New South Wales, Australia; in the Indian Ocean it is replaced by C. triangulum. It is marine and reef-associated at 5 to 20 m depth, and adults occur on lagoon and seaward reefs (FishBase).

Where Divers Find Them

At Gili Selang, off Amed in Bali, Diveplanit's account of one dive shows "a pair of Eastern Triangular Butterflyfish". It does not say how often they are seen.

FishBase says it feeds exclusively on the polyps of tubular Acropora corals, swims in pairs and is territorial; it lays eggs and forms pairs during breeding. Wikipedia says the same of its pairs, territory and diet.

Underwater Encounter Behavior

The sources cited here do not describe how the eastern triangular butterflyfish reacts to divers. FishBase lists it as harmless to humans.