The members of the family Pomacanthidae are generally know as angelfishes and, like their nearest cousins, the butterflyfishes, are regarded by many divers and aquarists as being amongst the most beautiful and majestic fishes in the sea. The majority of species occur on shallow reefs in coral, algae and sponge zones, most going little deeper than about 30m but where conditions are pristine and water is very clear, many species go much deeper and few species only live deep (over 100m). Angelfishes feature a large and distinctive backward-protruding spine from the lower corner of the gill-plate (cheek spine) from which the family name was derived. This cheek-spine is diagnostic for all the species , even at juvenile stage, and readily separates any angelfishes from butterflyfishes that may be similar in shape. Mot angelfishes are robust with compressed, ovate to rhomboid shaped bodies, covered with small or tiny scales, and have a continuous dorsal fin. The mouth is small and jaws are set with many small, usually tricuspid teeth that are used for grazing algae or scraping sponges and other sessile invertebrates. Few species combine their diet with a variety of foods and some are planktivores.
Northern West-Pacific, Japan to Taiwan, and southern Korea to Hong Kong, but mainland populations look slightly different. Found on rocky coastal reefs at depths from about 5 to 60m. It has electric blue stripes along its sides and a bright yellow tail. Post-larval juveniles are almost completely black, having a yellow band behind the eye that will disappear with growth. No sexual dichromatism is know and they occur in pairs or form small harem, and females are only recognised when gravid by their abdomen. It occurs together with a different colour form, similar to Chaetodontoplus chrysocephalus around Kashiwajima, western Shikoku and the Izu Peninsula, the sub-adults of which superficially resemble C.melanosoma and records of the latter are based on this form. It is not clear if this form is just a variation of C.septentrionalis or a separate species. Length to 22cm.