One of the most vividly coloured fish in the aquarium is a Yellow Faced (also called a Blue-Faced) Angelfish, Pomacanthus xanthometopon. Originating from waters around Australia north to Malaysia, they grow to be quite large, with a maximum length of 38cm. It is currently listed as Least Concern by the IUCN as a result of its large range, but threats to coral reefs from pollution, ocean acidification and other ecological changes may change this designation in the near future.
Currently there are at least thirteen species of Pomacanthus angelfish, which are distributed worldwide in tropical reef environments. Most species are variably marked with blue and yellow, though some are more monochrome. One distinctive feature is that juveniles are very different in appearance, most being marked with white stripes on a blue or black background. This may help to diffuse aggression between adults and juveniles, as adults are quite territorial and are usually observed in the wild either singly or as pairs. There is also ecological separation between the two age classes. In the Emperor Angelfish, P.imperator, juveniles shelter under ledges in the outer reefs, moving to progressively more open environments as they mature.
|P. xanthomatapon juvenile|
Food of these angelfish is varied, but is mostly obtained by pecking at the substrate, with a large component of the diet being encrusting sponges and tunicates. Juveniles may engage in cleaning behaviour, and have specialised swimming patterns to interact with their clients and prevent predation. Diet may change as they mature, with juveniles feeding on algae supplemented with detritus, crustaceans, and parasites, while adults switch to a sponge-based diet.
Although seen as adults as mated pairs, the roles of male and female are not necessarily fixed, as all known Pomacanthid angelfish are protogynous hermaphrodites, which means they start out as females and change sex as they age. In at least some Pomacanthid species this change is reversible, but whether this occurs in P.xanthometapon is not recorded. They spawn at dusk, rising up into the water column to release floating eggs. As with most marine fish, the larvae float in the plankton for months feeding on microscopic organisms before metamorphosing into juveniles and settling on a reef, possibly hundreds or thousands of miles from where their parents live. In captivity they can live many years, with 21 being recorded for P.xanthometapon.
They are popular aquarium fish, but in view of their large size and sensitivity to pollutants, especially nitrates and nitrates, they are definitely not beginners fish. In recent years specialised artificial diets have been developed containing the sponge proteins they apparently need for good health. Some species are being aquacultured for food fish, and at least a few species are also now being produced for the ornamental fish trade, but for now P.xanthometapon is only obtainable as wild caught animals, usually juveniles which are easier to transport and are more likely to adapt to life in the confines of an aquarium.
Next time, a fish with unusual shape rather than colour, the Orbiculate Batfish
Images from wikipedia