Chaetodon triangulum is one of 88 members of the genus Chaetodon, which together with 11 additional genera make up the butterflyfishes family (Chaetodontidae) in which a total of 129 species are currently described. This species is found in the tropical waters of the Indian Ocean and a small part of the Pacific Ocean. It is present on the east coast of Africa, from South Africa to the Red Sea, in the Arabian Sea, Madagascar, Seychelles, Andaman Islands, Thailand, Singapore, Indonesia and Taiwan. It inhabits coral regions at depths of up to 25 meters where it is usually associated with corals of the genus Acropora, as well as in seagrass beds.
The body of Chaetodon triangulum is triangular in shape, especially when the dorsal and anal fins are fully extended. The rostrum has an elongated snout, which ends with a small mouth. They can reach total lengths of up to 16 cm. The dorsal fin is unique and consists of 11-12 spines and 23-26 soft rays, while the anal fin has 3 spines and 20-21 soft rays.
The rostrum of Chaetodon triangulum is reddish in color. Behind it we find two black arcuate stripes, the first one crossing the eye and arising at the beginning of the dorsal and anal fin respectively. The second stripe is located slightly behind the first and the upper and lower ends are somewhat diffuse. Behind the pectoral fins, the body adopts a series of lines oriented opposite and diagonally from the central part of the body. These lines are white on a grayish background. The posterior part of the dorsal and anal fins are reddish, after which we find a thin black line and the edge of soft color. Something similar occurs in the caudal peduncle and caudal fin, which begins with a yellowish reddish band, to later present a wider black band centrally and the rear half of the caudal fin to be completely translucent, as well as the pectoral fins.
It is a territorial species that lives in pairs. Feeding is based on coral polyps (mainly Acroporas). They will defend an area where these corals are present, chasing without hesitation any other species or specimen of butterflyfish that enters their territory.
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