Angelfish

Marine angelfish are fish of the family Pomacanthidae. They are found in tropical reefs of the Atlantic, Indian, and mostly western Pacific Ocean. The family contains 7 genera and about 86 species. Among the family, we can find two branches: the “big” and the “dwarf” angelfish, depending on the dimension they can reach.
With their bright colours and deep, laterally compressed bodies, marine angelfish are some of the more conspicuous residents of the reef. They most closely resemble the butterflyfish, a related family of similarly showy reef fish. Marine angelfish are distinguished from butterflyfish by the presence of strong preopercle spines (part of the gill covers) in the former. This feature also explains the family name Pomacanthidae: it comes from the Greek poma meaning "cover" and akantha meaning "thorn".
Feeding habits can be strictly defined through genus, varying from species feeding on zooplankton and others preferring filamentous algae. Other species focus on sessile benthic invertebrates: sponges, tunicates, bryozoans, and hydroids are staples.
Most marine angelfish restrict themselves to the shallows of the reef, rarely venturing deeper than 50 m. They are diurnal animals, hiding amongst the nooks and crevices of the reef by night. Some species are solitary in nature and form highly territorial mated pairs; others form harems with a single male dominant over several females.
All marine angelfish species are known to be protogynous hermaphrodites: this means that if the dominant male of a harem is removed, a female will turn into a functional male. As pelagic spawners, marine angelfish release many tiny buoyant eggs into the water which then become part of the plankton. The eggs float freely with the currents until hatching, with a high number falling victim to planktonic feeders.
As juveniles, some species are more different in colours than when they reach adulthood. For example, the blue angelfish Pomacanthus semicirculatus is a vibrant, electric blue colour with black and white stripes or spots as a juvenile, but when it reaches adulthood, it turns into a greyish colour with yellow and light blue fins and dark spots on its body.
The reason why juvenile appearance is brightly coloured could be their role as cleaner fish, important on the reef and so well visible and recognizable. Others just have different colours from the adults to be well recognizable by the adults of the same species living in a common area, to avoid self-predation by them. Other juveniles have cryptic colours to protect them from predators.