A juvenile emperor angelfish is deep blue-black marked with a set of white and pale blue rings that curve around the rear of the body like contour lines. The adult is a completely different design: horizontal yellow and blue stripes running the length of the flank, a black mask across the eye, and a yellow tail.
The change begins at around eight to ten centimetres and completes over roughly twenty-four to thirty months, passing through an intermediate stage where broken rings and emerging stripes coexist. Several Pomacanthus species show comparably drastic juvenile patterns, and early ichthyologists placed some juveniles in separate species before rearing resolved the error.
The function is not settled. The leading suggestion is that the ringed pattern signals juvenile status to territorial adults, reducing aggression from established residents that would otherwise drive a competitor off the reef. Supporting evidence comes mainly from observation of reduced adult attacks on ringed juveniles rather than from controlled experiment.