Chaetodon lunulatus 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. The distribution of this species is very extensive, covering much of the tropical waters of the Indian and Pacific Oceans, from Japan to Australia, and from Hawaii to the Tuamotu Islands. Specimens of this species are found at depths of up to 30 meters, in regions with a large number of corals or semi-protected meadows.
With a markedly oval and laterally compressed body, Chaetodon lunulatus is a medium-sized fish, reaching up to 14 cm in maximum total length. The rostrum is short, with a pointed snout with a small mouth at its most anterior end. The dorsal fin has 13-14 spines and 20-22 soft rays; the anal fin has 3 spines followed by 18-21 soft rays and the caudal fin is slightly rounded.
The general coloration of the body of Chaetodon lunulatus is light pink in the dorsal area, turning yellow towards the belly. Several diagonal purple stripes, almost horizontal, cross the body. The pectoral and pelvic fins are yellow. The dorsal, anal and caudal fins have a thick black line with a yellow margin. On the margin of the anal fin, it has a reddish-brown stripe, hence one of its common names. The head is also yellow, with the black stripe across the eye, so characteristic of the genus, which in its case has a yellow margin. The mouth is black.
Its resemblance to the species Chaetodon trifasciatus is enormous. They are almost identical species in appearance, differing in that Chaetodon trifasciatus has the anterior part of the yellow-orange caudal fin and Chaetodon lunulatus has that white-blue part; and also in that the black bands that cross the eyes, in the case of Chaetodon trifasciatus do not come together at the top of the head, while in Chaetodon lunulatus they do.
Chaetodon lunulatus feeds exclusively on coral polyps. Reproduction of this species usually occurs just before dusk, when a monogamous pair releases eggs and sperm into the water column where they are fertilized.
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