Have you met the old woman of the ocean? The old woman angelfish is a remarkable animal. It is one of the rarest angelfish species, but also the only angelfish with dull colours - why?
The old woman angelfish (Pomacanthus rhomboides) is a very large angelfish, growing up to 45cm long. It is usually a dull brown colour with faded blue, white or grey stripes.
Despite these drab colours, the "old" woman has a surprise for us in its youth - displaying vivid electric blue and pitch black colours.
The main reason that the old woman angelfish is so rarely seen is its atypical lifestyle. Most angelfish are reef-associated, living near to rocky outcrops and coral reefs as hunters and scavengers, but the old woman angelfish has a different strategy: it forms large groups and hunts zooplankton in the mid-water, often far from reefs. It will also opportunistically feed on small crustaceans, worms, sea squirts and sponges.
Because of its tendency to live in open water, dull colours serve as better camouflage. It does not often compete territorially with other types of angelfish, nor does it compete with members of its own species, so there is little need for distinctive patterns to dissuade competition.
So why is the juvenile so brightly coloured? The juvenile old woman angelfish is commonly found in rock pools on the African east coast, but it is also often carried out into the ocean by the strong Agulhas Current. Its vivid colours help a juvenile to identify members of its own species in the deep waters.
It is a common feature of reef-associated species to view similarly coloured, sized and shaped fish as a threat, as many of them are highly territorial. The colouration of the juvenile old woman angelfish (and the juvenile individuals of many other angelfish), serve to differentiate it from an adult of the same species and other reef fish enough so that it is not perceived as a threat and can move freely about the reef.
Finally, the juvenile angelfish is in some ways more adaptable than its adult counterpart. In fact, it will often "clean" parasites off of other species of fish, and the vivid blue colours are a common signature of many different species of "cleaner" fish. The electric blue colours thus serve to signal its willingness to "clean" to potential client fish.